I've read that it's done more like escrow. The buyer creates a new wallet, the seller verifies the amount in the wallet, and then the buyer hands over the keys when the merchandise is transferred.
The seller still needs to make sure the buyer can't get a transaction transferring the coins back into a wallet under their control confirmed before the seller can move them out.
(I'm now vaguely curious about how the financial transactions on shipping container sized sales of guns/drugs work. But I'm not about to Google _that_...)
