BATFE is working to make sure you have one of their Licenses to sell a gun...You may not need one, but someone with one does to process the transaction.
Personally, I don't give a fuck if you sell one or one thousand without out any sort of Licenses or permission, but doing so could result in interactions with those that do and have the ability to put your ass in Jail...THOSE are the hearts and minds that have to be changed.