You can buy and sell to benefit your firearms collection though, correct?
You should worry about whether or not you're beginning to look like a business to the BATF about the same time you start to worry about not declaring your profits on your income tax form.
Selling a gun now and then for a profit is not going to be an issue.
Things that will get you immediate attention are things like buying several low cost pistols at once from the same dealer and then not being able to come up with them when the BATF drops by to talk a couple of days after the sale.
Or walking in with a guy who picks out a gun then turns to you to fill out the paperwork and gives you the money for the purchase after the sale.