You are wrong : seller _are_ paying. A fee
to list the item, a % of the final price and so 
on.

Why do you think that many people auction stuff 
there ? Sure, because it's a method of selling 
something. But it's also a way to let people know 
that the seller carry others, similar items.

Do you think that it doesn't count to potential 
bidder to know if a seller is specialized in a 
kind of item (especialy when concerning artwork) 
or not at all ?

Ads banner is nothing compared to what they gain 
on each auction. You seems to forgot that it's not 
free like Yahoo. You take an account to host your 
page with a paying provider and they add banners 
on the pages ? Who would use such a provider ?

Cheers,
Yann Stettler

PS: There is others regulation to prevent "empty" 
or "fake" auction.