unless the artwork is copyrighted, it would be very difficult to argue. Also if the seller copyrights it first then there is no chance of claiming any ownership.
Note that everything is copyrighted by default. It's not necessary to add a (c) or register it or anything. Anyway, some registration may be useful in case of disputes or to collect royalties. https://en.wikipedia.org/wiki/Copyright#Copyright_notice
If I make a bad drawing and I post it anonymously in imgur, I still own the copyright. If I draw it in a paper and I drop the paper in the street, I still own the copyright. If I draw it in the wall of a public toilette, I still own the copyright. ...
If I make a bad drawing and I post it anonymously in imgur, I still own the copyright. If I draw it in a paper and I drop the paper in the street, I still own the copyright. If I draw it in the wall of a public toilette, I still own the copyright. ...