You cannot sell so many guns that the ATF considers you to be 'in the business' of selling guns. For example, buying twenty Yugo SKSs and selling all but one wouldnt go over very well if you got audited, while buying a Century Arms U-Fix-'Em five pack and selling off the worst two or three would be acceptable (U-Fix-'Ems are sold as-is, and vary from VG with a cracked stock to rusted action and barrel with no stock, its all luck of the draw).
Kharn