Nothing is needed. You are already going to restrict out many of your more seriously interested buyers by requiring a BS, but that is completely your choice. There are guns for sale on this board right now that would have sold on the first day had they not required BS. You can search for threads that beat the subject to death.
The entire onus for the sale is on the buyer to not buy something if he's not allowed.
You are required to not know he's prohibited. You are not required to investigate him in anyway or see any ID.
One exception is if you can reasonably be expected to know he's underage, then you make sure.