I don’t think buyers should be able to enter a seller’s house without the seller’s authorization, but perhaps there could be a security firm that sent special “rent-a-cops” (pre-screened, with access to lockboxes) out whenever a buyer wanted to look at houses. The buyers could pay the hourly rate for these registered home-showers, and we could all live happily ever after. 🙂
edit: of course, the seller could opt for an “open” lockbox that (perhaps pre-registered/screened) buyers could open with a special code, if the sellers would prefer that (vacant house, or staged house with no valuables, etc.).