If 'pre-authorisation' was possible it would maybe solve that, considering that you need both authorisation and the game itself to actually play the game.
1) Seller preauthorises the buyer for that game (authorisation cannot be revoked)
2) Buyer sees that they're pre-authorised, sends the money
3) Seller sends the game
4) Buyer pops the game in and it works immediately with no further fuss
If the buyer doesn't pay up after pre-authorisation, it's no big deal since the sender won't be sending them the game, so that authorisation is kind of useless in this situation (where you're selling to someone anonymously over the net). If the seller doesn't send the game, then the normal eBay process would apply. Heck, maybe it could even apply as far as absent authorisation is confirmed in these cases, obviating the need for all this pre-authorisation stuff.
(I haven't thought through how unrevokable pre-authorisation could impact other types of sharing, so don't be surprised if this exposes loopholes or the like under other circumstances
)