Shops are just one model of transacting. But there are plenty of cases where sellers don't list their price and they have worked fine - think about the auction

Shops are just one model of transacting. But there are plenty of cases where sellers don't list their price and they have worked fine - think about the auction model for one. As an aside.. when do we use auctions? We don't use it for commodity items (no sense in buying a pack of candy in auctions).