Because it's not about who's the seller or who's the buyer, it's about who has a whole institutional structure set up for doing the thing and who is doing it casually. When you walk into a pawn shop or one of those tacky-looking high street gold buyer places, you may sell them your valuables or buy someone else's valuables, but either way the shop is the shop and you are the customer. You don't become the shop when the shop is buying from you.