Not so much for these games, but this conversation had me thinking about alternate mechanics for loot sales in the open source game I work on, and I think one solution is to have any loot of any value use more of a pawn shop/consignment mechanic. Rather than selling guns individually maybe you can put your crate of used weaponry up for sale on the black market, and then you have to wait for a buyer. Might take a long time depending on how much they're worth.