ATF has guidelines for what constitutes “Engaging in the business” of selling firearms, though has no concrete definitions on how many guns one may sell. The new “advisory” they just published as part of President 0’s new “gun control” push simply reiterates exactly what they already say.
Here is a layman’s summary of the matter:
To buy new guns at wholesale, one must have an FFL–Federal Firearm’s License, which come in several flavors for selling, smithing, manufacturing, ammunition, explosives, and with addenda for import or restricted National Firearms Act weapons–silencers, machine guns, short barrels, destructive devices and certain oddities.
If one plans to “Engage in the business” of selling firearms, new or used, an FFL is required.
by Michael Z. Williamson