Estate sales. I see used jewelry for sale at small shops, festivals, pawn shops, and flea markets. It's really likely that any small town with a used bookshop

Estate sales. I see used jewelry for sale at small shops, festivals, pawn shops, and flea markets. It's really likely that any small town with a used bookshop also has a used jewelry store. The ones I've been to sell a variety of other pieces as well. I think the OP is getting at the fact that people don't buy used jewelry from randos off of Craigslist. That doesn't mean there's no market for used jewelry, just that people are much more comfortable buying from a store or vendor, probably due to perceived reputability and maybe the fact that they may not immediately realize that it's used. This is especially true with pawn shops, where they may sell wholesale jewelry along side their walk-in acquisition pieces.