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.