eBay economics - total revenue impact from expanding ecosystem by 1 seller
eBay seems to think that additional sellers increase its profit (not just revenue), because it advertises the option to sell on its website and tries to recruit more sellers. That's the empirical answer. 

Theoretically, eBay could refuse to list the additional seller's product and not charge the seller any fees, so by revealed preference, eBay cannot be worse off with an additional seller. If eBay was forced to list every seller who comes along, then the revealed preference reasoning would fail, so then eBay may or may not be better off with an extra seller.