There are several DIFFERENT FFLS. An 01 is a Dealer, 02 pawnshop, 03 Collector of Curio & Relic firearms, etc. AND FFLs for importers, mfgr, mfgr of ammo, mfgr destructive devices,etc.
To be a dealer- you do not need a storefront- BUT- you must "intend to engage in the business" of buying and selling guns. That has been held to mean "devoting time and attention to making a livelihood". You cannot get a dealer's license for hobby pursuit. You must meet all requirements for having a business at the licensed premises- which CAN be your garage, basement, etc. You must meet zoning requirements, business license, sales tax ID, etc etc. If you are gunsmithing for pay, you need that license.
An 03 C&R FFL is for the hobbyist- the collector. Not a license to be "in the business". It covers all firearms that are 50 yrs old or older (in their original configuration) and certain other arms on the list published by BATFE. $30 for 3 yrs. No business premises. I CAN sell a gun to resident of my state, or FFL holder in any state. But should not be buying and selling so many to be a business pursuit.
Example- New Years Eve, a member here sent me an email- Dealer in Maine has a Mossberg 22 I might like (I collect Mossys) Called dealer New Years Day. Yes, they had gun, condition/ price what I was seeking- yes, they shipped- did I have a FFL for it to go to? Emailed them a copy of my FFL, credit card- 3 days later, UPS man standing there with my rifle. OR- walk into local dealer- handgun I want- made in 1960. We wheel, deal, arm wrestle, cut for high card, agree on price- I take copy of license out of wallet, sign it, hand it to him with cash, shake hands, take my handgun, drive home, enter it in the log book. Paperwork done. No 4473, no waiting on background check, no transfer fees. I can also order from wholesalers, and places like Brownells and Numrich give me a discount on parts.