No limits. 'Merica. Be careful and make sure there is no possible way to assume that you are selling them "for profit".
Selling "for profit" is not illegal and depends on market conditions. It you bought a Glop Gen 1 Model 3 for a FMV of $200 and some fool is now willing to pay $1000 for it, good for you,
Engaging IN THE BUSINESS of selling guns without the appropriate licenses is against the law.