If you're buying a crate of rifles with the intent to keep a couple or three and sell the rest, that is not in keeping with the FFL03 requirements. You are dealing in firearms and that is not legal.
If you're buying a crate of rifles, they all become part of your collection. If you later decide to keep three and put the rest aside to trade or perhaps sell to others over time, in order to acquire other C&R collectibles, that's fine. Buy one crate and get rid of the excess over a year or two, that's alright. Buy a crate each month and rent a table at the shows all year around, you are violating the FFL03 requirements.