>They can't ban everything, since at the extremis you could buy crypto from eBay sellers.

This is against eBay's policies. All items sold through their site must be delivered physically. If you type "bitcoin" in today you'll only get hardware or novelties, not the coins themselves.

Okay, then sell a hardware token with the bitcoin on them, or the coins as QR codes printed on artisinal paper or whatever.

Or do the sale on some other platform that is similar to ebay.