I work at a smaller studio and we do buy and use other people's art from time to time (whenever it can save the artists some time without compromising quality). It's usually the gui packs (buttons, menus, etc) that are used exactly the way you describe.
With art it's a bit trickier - your art must be non specific enough to make different game developers want to use it (e.g random cute chibi girl will be better sold than a steampunk lady fighting a robotic dragon on mars).
Another thing for you to consider is that you'd probably have to give the studios the right to edit your artwork as they like - I know we edit the assets we get beyond recognition (so they would fit our style).
Aside from that, I think it's a good idea, even though I'm not sure whether it would turn out profitable for you.
(btw I'm just giving my opinion as a regular person, not as a mod - I don't have the power to make the call 
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