A FFL is not required to buy guns- one is required to "be in the business" of buying and selling guns to make a living.
Now, how are you going to be sure you are not buying a stolen gun? Yes, there are ways to reduce the chances of that- but you need to give that some thought.
As far as prices paid for used guns- 1. Most people think that what they have is worth a lot more than what it is- and 2. If you are running a business, you have to pay taxes, rent, light bill, salaries- and make a profit. How long will that gun sit in the case, tieing up money I paid for it?
BTW, other than a Collector's FFL (03 C&R FFL) you will not GET a FFL unless you intend to "be in the business". And before you can be issued such a license, you will have to meet all local and state laws for running a business- business license, zoning, sales tax ID, etc etc.