--Could I sell this image without being sued by Nintendo?
Possibly. Quick, I'll give you $.25 for it? Done...you just sold one virtually and no sirens do I hear. Check your paypal.
--Could I sell this image and be sued by Nintendo?
Yes. But can you sell enough (or whatever) to get noticed by them as a counterfeit pokeyballer guy worth their time? I've no idea, we just met.
Is this also a possible trademark issue? Maybe, that ball may be registered trademark.
Sometimes there is no copyright registration on an item like this. Copyright is basically automatic, but registering a copyright is manual process and needs to be already done when the infringement occurred, to bring action for that infringement. (No reg, no sue, and no retroactivelys).
Most important thing about all this though...
..................................................Squirtle!!!!!!!!!!!!!