That would be interesting, as cards used through Apple Pay only work if the card network is accepted (i.e. you can use Apple Pay at Costco, but only with a Visa card, it doesn't get around the Visa-only requirement for credit cards). If that's the case, I wonder if eBay is merely preventing one from adding Amex cards manually and not reworking their entire system. This is sort of like the in-person merchants that "don't take Amex" but it's more of a policy than a capability—their terminals can read and process Amex cards just fine.

PayPal and Venmo make sense as they obfuscate the original card in the process—also why certain things like Amex offers or merchant coding don't work with those.