It’s not that shopping for vintage tees online is hard. It comes with the exact same risks of buying any article of clothing on the internet, with those risks cranked up maybe 20%. Cotton shrinks with washes and fabric warps and stretches with wear. Unless you’re picking up a deadstock tee, you’ll have to factor in the way these things age, which makes finding great garments a little more complicated than just plugging in “large” to the size filter on whatever platform you’re using—be it eBay, Grailed, Depop, or Poshmark. All things considered it isn’t that hard. You just have to know what you’re doing. In most cases that means accepting that trial and error are parts of the process. Still, if you want to get ahead of an error or two (and save yourself the wasted cash in the process), here are some good places to start.

Go by measurements, not shirt size.

This is the big one. If you want to build a great vintage tee collection you have to start by forgetting about traditional size charts. Tees stretch and warp with wear and shrink with washes. On top of that, you have to factor in everything from the branded blank tee the shirt is printed on, the year the shirt was produced (blank measurements have always been inconsistent over the decades), and whether it’s a thin tee or a heartier cotton.

Or, rather than treat buying a tee shirt like an algebraic equation, just know your measurements instead. The best thing you can do to make buying vintage online a little easier is to go to your dresser and find your favorite T-shirt—the one that fits you the way you wish all your shirts did—and take the pit-to-pit measurements, as well as the shoulder-to-hem measurements, and either write them down or commit them to memory. Any vintage seller worth their salt will include these measurements in listings, and sometimes even in the photos. If they aren’t there, it’s easy to ask for them. It’ll save you the trouble of spending, say, forty bucks on, hypothetically speaking, a Jimmy Buffett shirt that would hardly fit an American Girl doll.

Know your blanks.

If you’re looking to get really nerdy about it, the way to supercharge your vintage shopping skills is to take some time to research T-shirt blanks. It’s a research rabbithole for the detail-oriented, one that’ll have you developing strong opinions on decades of Fruit of the Loom’s quality, cotton blends, and stitch standards. “Blanks are definitely an important factor [for me],” says Öner. “They can show whether a shirt is original, though you still have to watch out for fake tags.” He also namedropped early-’90s blanks from brands like Fruit of the Loom, Oneita, and Screen Stars as favorites, knowing that they’re made of thick cotton and wear the way he wants them to.

Don’t just take Öner’s word for it, though–or anyone else’s. Figure out which blanks tend to fit you and feel the best, and then seek out shirts with those tags. Plus, when it comes to authenticity, knowing a specific Nine Inch Nails tee that’s listed as being from the ’90s should have an AllSport Pro Weight tag and spotting that the one you’ve been eyeing is on, for example, a suspiciously new-looking Comfort Colors blank, could save you some wasted cash and a pain-in-the-ass refund request.

Understand what you’re buying.

Buying vintage tees isn’t rocket science, but like almost anything else, it’s easier if you know more about what you’re doing. This doesn’t just mean researching T-shirt blanks and combing through eBay listings–it means understanding what it is you’re looking for and also why you’re looking for it. Öner suggests that if you’re really set on a particular true-vintage tee, you would do well to look up photos of the original shirt to get a feel for the specificity of the print and what the OG tag looks like.