> eBay still accepts ... PayPal, Apple Pay, Google Pay

Which means they still accept Amex, but through places that one would imagine add their own processing costs on top of the fees they've negotiated with Amex.

It'd be really curious to see the costs involved here.

Would they? Apple Pay and Google pay are just proxies for the actual cards, so if a play does not accept a type of card, that will also be true for the virtual card presented by Apple Pay.

I agree about PayPal, but they charge their own fees and abstract the source of the money away completely. Presumably they have a better deal with Amex

Buy eBay gift cards at a grocery store on an Amex with a grocery multiplier. Get more points and still use Amex.

Wouldn't you lose consumer protection entirely going that route? You'd get more protection buying directly with Visa or Mastercard.

Only if by "consumer protection" you are OK getting your Ebay account banned. Ebay already provides there own buyer protection... but if you disagree with them and do a charge back then most likely your account will be banned.