If you commission art from me, you can print it on whatever items you'd like, just no reselling the items unless you contact me first.
You can however "sell my art" if it is in a book or graphic novel, whether as the cover or as illustration, just please credit me and contact me first (I'd love to check out any products you make!).
Other than that, you can use my art for whatever you want as long as you credit me and use it for personal reasons.