It s not illegal for you to sell a gun that you build, so long as you're not in the business of making and selling them. If you want to sell it, do it. The usual conditions for selling a gun would apply, such as the buyer being 18 or over, and any state/local laws specific to your area.
Only problem is interstate sales that have to go through a FFL dealer. You will have to find one that will handle a home made gun. Some will not. Harry
That is how my TNW MG34's were listed on the ATF form when the came in. Same for the Wiselite M53 that I purchased a few years ago. All long guns as they are semi-auto rifles now. Harry