Affiliate commissions. eBay
both pay part of the site bills around here.
I get the cheerleading for hipstamp from a purely anti-eBay perspective, but from a realistic perspective (mine at least), Hipstamps is the Land of No Sales. I tried selling there multiple times over the years and the sales there were a tiny fraction (as in less than 10%) of what I sold on eBay
A bigger problem, in my opinion, is the structure/layout of the item pages on Hipstamp. It is an abomination.
P.S. @91stang: The fact that you didn't read the fine print in what you were agreeing to is somehow eBay