I might be wrong, but I'm pretty sure the German government has already told them that they have to, at least so far as their own citizens are concerned.
Regardless, IF they do go that route, I imagine it'd be Steam wallet transactions only. That's a win-win for Valve and for the publishers at least it'll mean there's a chance some of that money will end up being spent on their new games. I can also see them putting a limit on how soon after release a game can be resold (I'm thinking 6-8 months?) so as to not hurt initial sales. From what I gather that's more-or-less the crucial period in which most profitable games make back their money.