As with Jered Weaver's fractured elbow, Zack Greinke's fractured collarbone is unfortunate, to say the least, but you really have no other choice than to stash him if you own him in Fantasy.
He's going to miss several weeks, and that sort of timetable will scare off most prospective buyers, leaving them to make half-hearted offers that don't account for the full scope of the season.
Selling Greinke for 50 cents on the dollar only compounds the loss by making it permanent. You don't have to win the league during the time he's out. You just have to keep it close. If your team isn't good enough to stay in the running without Greinke, then a pitcher half as good as him won't be enough to save it. Perhaps you should try selling off other parts instead.