Do they have DRM or something? I hope not. But if it doesn't, what's stopping anyone who bought the asset from uploading it somewhere else?
This is an issue with open source app/resource stores that to my knowledge no one has solved. If you stay true to the Free (as in freedom) software philosophy, then you can't really put anything in to enforce paid access to something, and even if you do, anyone with a text editor can just take that code out. But if you just let anyone who buys it redistribute it for free, you're not going to attract many sellers because they wouldn't trust their content to remain paid access only. Add to the fact that paid content is inheretly proprietary, or at the very least, the author certainly wouldn't choose to put Free as in freedom licenses on their content because that would literally legally allow anyone to redistribute it for free.