Yes, in most states, a private sale is legal. However, if you are charged with "Engaging in the business" of buying and selling guns without a license, you ARE going to need a lawyer. A pattern of buying and selling will do that.
No, no magic number of guns. No, no magic time you must hold a gun before selling it. In some cases, it has been the purchase and resale of ONE gun.
You collect military arms. You have an M1 Carbine. You buy a better one, decide to sell the first- NOT "engaging in the business".
You go to a gun show- really good price on 91/30s. You buy 6 of them, keep one, and over the next month sell the other 5 to your buds. "Engaging in the business".
You are moving to Fresnoistan where you are not permitted to own .29 caliber rifles. You decide to sell your collection of .29 caliber rifles. NOT "engaging in the business".
Please remember legal advice posted here is worth every penny you are paying for it. I am not a lawyer, and even if I were, I am not YOUR lawyer.