I've gotten used pixels off of ebay more than once and never had a problem. I feel like if it was that simple swappa.com wouldn't exist though. Maybe I've been lucky?

Same here, I bought my current smartphone, a Pixel 6, two or three years ago for 200 dollars on eBay, and it arrived in pristine condition. Super small bump on one of the frame edges, not a single scratch on the display though now there are many, of course, I'm not that careful and the phone is cheap enough not to be too upset.

I generally trust private sellers a lot more than professional ones, if you vet them well enough (check their page, check their reviews, check their other listings), the chance that you'll get a good deal on a device in the condition described. The incentive to gain a profit is a whole lot lower, a commercial seller has to make sure they're the winner in some way during a transaction, they have bills, staff and assets. A private person is often happy to just get rid of the phone, as it would usually rot in the old phone drawer, until it eventually ends up in an e-waste bin a decade later when they clean out. Getting a little money back is already a win. Surprisingly, I've never had a bad experience. I presume there's also buyer protection, but I'm hoping I never have to use it.

I've also had good experiences on eBay. Buyers can leave bad reviews for anything so good reviews are a strong indicator. (That said, there are plenty of "genuine" laptop batteries for $15 from high-scoring sellers.)

I feel like eBay put a lot of work into creating a competitive marketplace where honesty was rewarded, and it basically works well but they got an unsavory reputation anyways. Then Amazon tried to hide that complexity from the end user and buyers just get burned. Maybe Swappa is trying to go the Amazon route? I do not think there are real shortcuts here. Either the marketplace needs to vet sellers manually, or offer a competitive, transparent reputation system like eBay.

I think eBay's larger volume makes it easier to find a seller that will go through the "infeasible" trouble of sorting OEM-unlocked devices. I very recently switched to Graphene and nearly purchased from a listing that had similar "may or may not be OEM locked" language before finding one that promised it was unlocked.