It really amazes me the number of people who have a lot of misconceived ideas on a subject. Reading above I have found many incorrect statements concerning this subject. There is no "certain number" of guns that must be sold. Many gunsmiths hold FFL's and don't sell many firearms, but must have one to accept a gun into their business just to work on it. You don't have to be zoned commercial, just not zoned against commercial selling. You don't have to have Law Enforcement permission, you must notify them when you apply for FFL but nothing else has to be done. I don't even think they can refuse your application to the Feds unless they advise the Feds that you would be inviolation of a local zoning ordinance. Really don't have to have a store front or "regular hours", you do have to put down at least a common time to allow them to make a compliance inspection no more than once a year, but most dealers do not even see an agent in 10 years. As far as I know,there is no background check, the law says that the BATF "MUST" issue FFL within 45 days (I think it is 45) after you apply unless they find you do not comply with basically what you would sign on a 4473. Only one set of books is required, and if you "take" a firearm for yourself, you simply note in book that it is transfered to your personal collection and it can even be stored with for sale firearms if it is properly marked "not for sale". Only hitch here is that if you personally sell it within one year, you must note it back into your book and have buyer go through same proceedure as purchasing firearm out of FFL inventory.
I really enjoy hearing people speak of what they do not fully understand. Almost as much fun as hearing people speak of NFA ownership and having to have a "license" or pay a fee each year. Not to mention that full auto is "illegal". Did you know that in the USA there is no such thing as an "illegal firearm"? The proper term is "prohibited" since every firearm is legal to own under certain conditions.