Well, the companies can set up royalties, so any time a user sells an item, the company gets a x% cut (customizable). If you have some big ticket items or lots of smaller items in your game that could switch hands a lot, could be an easy revenue stream.

On "supporting everything, not just your own company's games" - I agree that supporting everything is too much. Take a look at the Amiibo ecosystem though. If I have a Zelda Amiibo, I can (probably) use that in any Zelda game to get some free items or skins. There's a chance I could also use it in a completely different game (lets say Animal Crossing) to get some new skin or item in there as well.

And the best part is, indie companies can do this too. "Hey come play my indie game, and if you happen to have XYZ item from WoW, you can start with 1000 free gold!" Or y'know, whatever. But it should open up a lot of new avenues for sharing data across games.