The licensing at the store says that commercial uses, so long as the original file can not be extracted, are acceptable. One merchant has tried to screw with that (multiple times) and has been taken to task, but the store license that you agree to before purchase explicitly says that you're allowed to use the items in commercial ventures. Extracting files means things like creating a flat plane and "texturing" it with a Mike texture, for example, and posting it. Anyone could then copy that picture and have a working Mike texture. Redistribution is also prohibited. If you can make a buck off of something that you purchased at the Marketplace - go for it.