Income. Big stores = big money + eBay/PayPal tax = eBay profits.

Small store or non-store seller moves half a dozen 99c items a week, eBay doesn't get enough money for a roll of toilet paper. Ignoring the fact that there are literally millions of people buying and selling on eBay worldwide.

Of course, if eBay drops the 40 free listings per month, my account will go back to sleep for another decade, since I rarely buy anything on eBay.