The thing about eBay is that they have succeeded in creating themselves as a "pseudo-monopoly" in their particular business space. So they can get away with it because people don't really have many viable alternatives.

My wife uses primarily Poshmark and has occasionally used Mercari with mixed success, but her gig is clothing and home accessories; mine is stamps and paper collectibles which is a very weak (to non-existent) category on Mercari, sadly.