A few things that apply across the board:

Thrifting takes time. Do not expect to always find something.

Write down all your measurements: measure your body as if for tailoring (guide when you don't have someone to help) / dressmaking, measure your favorite clothes. Measure the spots in your house that could use furniture/art. Measure how much space you have in your car.

Keep a measuring tape in your car / on your person. Same with any color samples, etc, if you're furnishing your home. You never know when you may spot a shop or sale to pull into.

Learn what can be fixed/altered with the skillset and resources you currently have. Start off by buying what you can live with as-is.

Buy for yourself. Once you develop some thrifting skills, only then should you consider flipping items.

You cannot rescue everything. Again, buy for yourself and what you honestly expect to use. Resist the urge to hoard!

When I suggest searches, your starting places should be facebook, search engines, and google maps. You are searching for both in-person and online resources. Once you find a place in person, ask around for recommendations.

There is more out there than can be reasonably hoarded or sold! Seriously, chill out at the thought of resellers. There's so much out there. Old != rare.

On to second-hand shopping! These are my own categories, and there aren't always clear lines:

Thrift stores

Thrift stores generally sell donated goods. General wisdom is that you can find better stuff at thrift stores in high income areas, but stores like Goodwill will move inventory around, so that isn't always the case.

The big national stores are Goodwill and Salvation Army. Both are shady, you can do your research here. There are also regional chains, like Savers. A thrift store may be for-profit or non-profit.

Chains may have store tiers: for example, Goodwill has regular stores, "boutique" stores (higher end clothing), an online store/auction site, and warehouses where you sift through bins to buy by the pound.

A warning: Big chains are increasingly selling overstock items from cheap manufacturers; most of it is garbage, imo.

Tip: It's a common practice for thrift stores to have a tag color of the week that is heavily discounted -- this helps them move old inventory.

Smaller thrift stores are where I find a lot of gems. Many are run by churches, hospice facilities, senior centers, and humane societies. Keep your eyes open, or do a search on Google Maps. There are also niche stores -- I've seen art supply and fabric thrift shops.

Consignment / resale stores

These stores buy their merch from people and resell it. When buying, they may give cash or store credit. Some stores work on a consignment model, which means you don't get your money/credit until your item sells.

A pawn shop is the prototypical resale store. So is a used bookstore. Antique stores usually are, too, but not all actively buy from walk-ins.

You may be familiar with some chains: Play It Again Sports, Plato's Closet, etc.. Look for your local stores as well as non-chain online stores that suit your interests. (I'll be adding some vintage clothing stores next time I update my wardrobe recommendations post.)

You may also get lucky at places that do repairs: cobblers selling shoes, vacuum/sewing machine repair shops selling machines.

Salvage stores

Salvage stores sell items they have found for free, bought in lots at auction, or had donated by corporate sponsors. Many of these stores are specialty stores, but some are indistinguishable from thrift shops.

Habitat for Humanity is a big one for building supplies and furniture.

Many towns have a salvage furniture store. C&D Secondchance was a favorite of mine when I lived in Central Florida. These places will literally have $20 nightstands and chairs stacked to the rafters.

A lot of junk-hauling places may have a store -- these can be amazing to visit. Same with any junk shop with a zero-waste philosophy. If you're in G'ville, Florida, the Repurpose Project was a fav of mine.

Architectural salvage is also a very common sub-type here -- it's where you can buy columns, doors, windows, doorknobs, etc. But they have keened onto trends in home decor and tend to jack up prices.

You can even find salvage grocery stores -- they are where seasonal breakfast cereal and dented packaging go for one last attempt at a sale. You'll learn quickly that "best by" dates are usually just a suggestion.

Generally, things from salvage stores will require some elbow grease to get looking nice. But not always. Unclaimed Baggage is a great exception -- they sell items from lost luggage.

Surplus/overstock/liquidation

Surplus stores sell either items that are new, but no longer on the open market. Or, they sell items that an organization no longer needs.

Army/Navy stores are commonly thought of as the typical surplus store, though their quality varies. Stores like Ross are national chains that are overstock stores.

Large organizations use surplus stores to liquidate their old assets. Your local government, library, or school system will likely have regular sales and auctions. Or a commercial store may be going out of business. Search online for what is near you.

Home liquidation

These are your garage/tag/yard/moving sales. Generally, these are items a household no longer has a need for.

Announcements are often placed in facebook, craigslist, local papers. Look for signs posted around neighborhoods. And many communities do group sales, where they encourage everyone to have their garage sale all on the same day.

This category also includes estate sales, which is the selling of assets after a person dies. Most of the time, these are run by companies that specialize in this market.

Estate sales are pretty amazing - they are a time capsule of a person's life and you never know what you will find. I have gotten everything from paintings to cleaning supplies. If you are a sewist, estate sale curtain liners and other linens are fantastic for mockups. Announcements are similar to garage sales. Be prepared to brush elbows with resellers on the first day. Prices are typically slashed by the last day. I have been to some sales that were so poorly attended and so time-sensitive (the house was being immediately sold), the family just begged me to haul away (for free) as much as could fit into my car.

It may feel ghoulish to buy a dead person's stuff, but it's typically a huge relief for the family to see it all go. Trust that the family has saved the sentimental items -- you are doing a favor by taking home all the mundane things like silverware vs it going in the trash.

If the family is on-site, be respectful. I have found many are delighted to know that their loved one's items will go on to be treasured by someone else vs going to a reseller.

Many estate sales are also held online and offline as auctions. Auction Ninja is a popular site, though many others exist.

Scratch-and-dent / outlet stores

These stores sell new items with cosmetic or other minor imperfections. These are not always standalone stores -- they may be a section inside the store, like IKEA or many furniture stores. This isn't quite the secondary market, but adjacent.

Scratch and dent is going to be hit-or-miss, especially if it's directly from the brand. Buying the floor model of a piece of furniture is not quite the secondary market, and imo, you are better off buying used.

I do not count outlet stores even in this fuzzy category -- that is because goods are increasingly being produced *just* for outlets. It is no longer the case that this is where factory seconds and seasonal stuff gets funneled to. Stop shopping at outlets -- you are not getting a deal and the products are intentionally inferior.

Direct sales

This is the big one. You are likely going to get the best product and prices directly from an individual.

There's lots of sites out there for buying/selling stuff online - Poshmark, eBay, etsy, Depop, Vinted, etc. And lots of ways for stuff in-person: Facebook Marketplace, Craigslist, etc.

A special note: Instagram has become increasingly popular for people selling niche used goods, especially in the vintage world.

Many of these places are driven by user reviews, so learn the etiquette and any group rules. Remember that this isn't Amazon, so be kind when sellers need more than a day to ship to you. And when you by in person, being on-time and paying with cash are king.

When buying online, learn the jargon. NWT (new with tags) and NOS (new oldstock) are common abbreviations. Develop a good eye for reviewing photos for quality -- hunt for flaws. And don't be afraid to ask for additional photos or info, such as measurements or if there's enough seam allowance in a garment for an alteration.

Because there are so many online sales, I like gem.app to search across multiple sites. But even this tool barely scratches the surface.

There are also niche communities that buy and sell amongst themselves. My vintage menswear group on Facebook does this. Many vendors - especially those who do frequent, small drops - have a facebook group for the secondary market (My partner is part of a makeup group just for Notoriously Morbid.). And large online forums for special interests often have a buy/sell/trade sub-forum. (As a leatherworker, I frequently visit leatherworker.net's marketplace)

Freecycling / buy nothing / swaps

This is where you can get items for free, and sometimes trade. Lots of seasonal and kids' stuff. Petfood that kitty refuses to eat. Pantry goods that haven't expired yet. That sort of thing. Check Nextdoor and Facebook for local "buy nothing" groups.

You may also have some in-person.

There are also groups that do swap meets. As a trans man, I'm mostly familiar with LGBTQ+ orgs hosting clothing swaps. But many specialty hobbies also do similar,

Events & markets

Holiday markets, flea markets, swap meets, antique shows -- the lines for all of these have increasingly become blurred. Check your local events for what is near you. Some of these happen like clockwork on a regular cadence, and some are popups.

Don't be afraid to attend events outside your norm -- for example, I am not religious, but enjoy going to church rummage sales.

The "Antique" fair is a wide category these days. I've seen crafts, imports, vintage clothing, and 100+ year old furniture all at the same event. If you are in the New England area, I highly recommend Brimfield. If you are in Florida, the Renniger's shows.

Again, learn the etiquette. Antique fairs generally encourage haggling, but still feel out the individual vendor's vibes. I usually just hold up an item questioningly and see if they throw out an offer. On the flip side, crafting vendors will find it incredibly rude if you suggest a lower price for their wares.

Multi-day events tend to have predictable rhythms: as the days wear on, prices go down and vendors start packing up. I generally go early in the run, and also on the last day.

Wrap up

I'm sure I'm missing some resources. I know I'm short on individual links -- this is where you'll need to search on your own, based on your location and interests. (And please people - feel welcome to drop in specific links to your favs, even if they are regional.) The secondary market is huge!

And remember: Thrifting works best as a routine. I probably browse online daily and in-person several times a month.

To avoid being frustrated when you can't find something: treat thrifting as visiting a weird, ever-changing museum where you occasionally find something super cool.