The categories for this are already ingame. Just look at https://rec.net/image/2271527
Then it will probably come
the problem with this is some people are having a problem with people stealing inventions and taking credit for making them. Allowing sales will only make this issue worse.
@joe frechette: well then maybe there could be some sort of system in place that allows you to report plagiarism.
@Wick: Or just give the inventions away for free if the creator chooses to do so. I don't think we need incentives for inventions. There are already people making many rooms out there for fun.