Well, yeah, it's normal for a shop that's asking you to pay for something to explain what the heck you'd be spending your money for. I was asking about deals in the other direction, which only operates if people can also sell things to the shopkeeper.
One of the games people play with ID is to walk into a shop and say, "I went for a walk and I found this stick." And if the shopkeeper offers you 342 zorkmids for it you know it's a pretty good stick. Like a wand of bunny-flinging or something. On the other hand, if the shopkeeper looks at you funny and says, "Dude, that's just a stick. Are you mental?" You figure, okay, so, it's nothing special. But if the shopkeeper offers you 7455 zorkmids for it, you know it's a really awesome stick, lke a wand of elephant-flinging or something.
Anyway, in a lot of games it develops that the shopkeeper offers you money for it without telling you what it is, but you can cross-index the amount of money with a spoiler to figure out what it is.