As Renegade said, there is no limit.
If you willfully buy a firearm with the intent to make profit, it requires an FFL on your behalf. It is held that you are engaging in commerce.
Now if you buy something, then decide that you no longer want it and turn a $100 profit, then you are not engaging in commerce.
This is what nailed Copeland in Austin. He did not have an FFL and was buying from other tables, and selling the items on his for profit. He was engaging in commerce as a means of lifestyle, and that is illegal according to the ATF.
This is what I was thinking about....