It is true that some shops only buy certain items. But after awhile, all the shops that would buy the item will not buy them anymore. Even some shops will not even buy back what they just sold you. If you'd like us to "bug" the shops that do not buy back the items they have to sell you that you want to sell them... plz let us know...
Poobah should probably offer the more difinitive answer for you, but my suggestion is this.
First, go ahead and bug them, it's always good to point out stuff that seems odd. Poobah's good at fixing the reported bugs before he does anything else.
But second, in this case, I very much suspect Poobah's up to something plot-like. I hate putting words in my colleague's mouth, but I think not all shops are willing to be buyers, and the ones that are willing are subtly flagged with some signal, such as "Allworthy displays his new (and used) wares." The "and used" is maybe your clue here.
Re not buying things anymore, the explanation is simple: the shopkeepers run out of money after a while. They need to sell things, too.