As long as they're marked as sold, I don't have a
problem with them.

Then again, some of my cels are still on dealers
sites and I don't mind. One of them has been up
for nearly three years after I bought it. (From
the person who bought it from the dealer) 
Fortunately, it has been marked as sold for all
that time.

If it bothers you to have it on display at the
seller's gallery, I think its fine to let them
know. Its more traditional to not display items
you don't have anymore, even though I enjoy seeing
them. Its not worth it if it makes people unhappy.
However, since they're getting more common, maybe
there should be some sort of set routine for them.
 Such as, always asking the buyer beforehand
whether or not they mind.