I don't get why companies like this can charge a percentage equal to the credit card fee. If you pay 6% + $0.30 (on low ticket items, $0.30 could add additional

I don't get why companies like this can charge a percentage equal to the credit card fee. If you pay 6% + $0.30 (on low ticket items, $0.30 could add additional few percent), but your margin is, let's say, 20%, you literally need to pay a third of your margin for processing. If you sell your old shoes or something that doesn't cost you anything but your time, then it's okay, but all these marketplaces literally are saying: "Low-volume low-margin merchants are not welcome!"