If I were a rough government agency actively supporting a wannabe dictator, my interpretation of the presidents remarks regarding "unlicensed gun dealers" would cause me to have a letter/email ginned up stating in the most hostile tone of voice that it is illegal to be engaged in the business of buying and selling guns without an FFL. Note that I did not define "business". Let 'em guess. Just copy and past the whole statute into the letter in paragraph 1.
Paragraph 2 states that if the recipient is in fact a licensed dealer, then please reply with a copy of their license and a valid photo ID.
Paragraph 3 states that if the recipient is not a licensed dealer, then they must reply with a complete explanation of why they would not be considered one with supporting documentation and a copy of a valid photo ID. Documentation should consist of the make, model, and serial numbers of any and all firearms purchased or sold in the last year. If you are not doing business in firearms, please include photos and descriptions of the merchandise you are selling and a determination will be made as to the validity of your claim. Buying and selling small numbers of firearms for your own use is acceptable. File your papers within 14 days and we'll tell you if you're going to jail.
Thank you for your cooperation in this matter.
Bureau of harassment.
This letter/e-mail would be sent to everyone with a posting on a gun brokering internet site and all purchasers of tables at all gun shows.
I doubt that this would slow down the sales of guns any, but I'm sure it would reduce the number of background checks the FBI is having to perform as all private sales would go underground and gun shows would disappear.