When I was trading at the Drunken Huntsman (archery supplies) I noticed that Auriel's bow didn't show up on the list of sellable weapons. When I afterwards traded with Belethor the bow showed up on the list, though. Is this some kind of bug or isn't Auriel's bow sellable to everyone?

Quest items should not show up as being sellable until after the associated quest is resolved.

I finished that quest long ago.