Initial license is $200 for 3 years. Renewal is $90 for 3 years. Hardly too expensive. These FFL horror stories do make the circuit. Let me tell you if Walmart, Sportsmans Warehouse, Academy, Cabelas, and any other chain that sells guns (with the numbnuts that sell them) can stay out of trouble, so can the small home based dealer.
The ATF is not sweeping down and arresting you in the middle of the night. Nor doing inspections without notice. They do have the right (since you sign it away) to inspect the premises during business hours. Or by calling and making an appointment. They also can access your records for legal reasons as needed. They do not care if you have a storefront, or a separate entrance. They do not care if you make a profit. They want you to conduct legal transfers and maintain proper paperwork, that is it. Yes you must tag any guns that are "for sale" or "not for sale" and they can be in the same safe.
The inital process will take a long time. You must maintain that you are getting the license to run a business, not create a collection. I have never met an ATF enforcement agent, just a office paperpusher that seemed to know little of guns but knew a lot about the laws about guns. He provided me with a "how to" run the paperwork and secure the weapons and inspected the area. You must provide proof that you are zoned for a business and get a business license and notify the chief law enforcement officer for your area of your license.
The gun business has gone volume discounts, so do not expect to make a large profit selling guns. I can buy guns retail usually for cheaper than I can get them wholesale off the internet. I do not have to pay transfer fees or tax. I ordered a Stevens 200 from a wholesaler and with shipping and handling it cost me $260. Last time I was in Sportsmans Warehouse they were selling for $260 plus tax. Hardly a profitable venture...