I don't see a really effective way for a third party to deal with this. However, if you want to do something I'd highly suggesting posting on the modthesims website forums. If the skins are by another creator and being used without consent they are liable to recognize the original creator and let him/her know about the infringement.
If the person advertising the skins on the marketplace is the original creator there isn't much to be done, alas. Like has already been mentioned, skins are always a risky buy since they rarely look very good on any shape aside from the one they were designed on. Hopefully most folks are savvy enough to look for a demo first.