Selling inventory that isn't: As long as the seller makes it clear that this is the case, I don't have a problem with it. When I go to a bike shop to buy something, they often special-order it.
If somebody finds a stash of stuff, but is afraid to buy because they don't know what the market value is, I don't mind if they propose the items, as long as they say "I think I can get this. Are you insterested?" Of course, publicly advertising something for sale that you don't own is a different story.
In fact, the René Herse book order I am placing on Monday is like that: Almost all books I order are sold already.
Jan Heine, Seattle