IF you are selling GUNS for a profit for the purpose of being in the GUN business, you need an FFL.
In today's world, you need a business location, in compliance with local zoning and business regulations.
Go to ATF's website for basic info. You will have to notify the local police/sheriff and be in compliance with all local/state/federal business license, tax, zoning regulations within 30 days. ATF will background you and an Agent will visit you and interview you before the issuance of a license.
# to 6 month process is the best guess.
Casual resale/trading of guns is OK as long as you make sure you are not selling to a prohibited class person and not violating a state law. No FFL is required.
DGUNSMITH Good, Safe Anwser there! Too bad it's far from ther truth (In my Opinion!) The ATF has No Rule for How many Firearms you need to sell to Be Needing a FFL! They dance around the issue and they never give you a direct anwser! They will tell you 3 a month one time, 4 a year the next! Tell me I'm wrong? In this case, they try to use that Agent Discression crap, and skirt any Direct anwser! If you doubt me, Call the ATF and ask them the Direct Querstion, watch them spin thier tales and Yarns? LOL Remember, your not dealing with folks that Play by the rules but always expect you to do so? In short, thier are good agents, in the ATF as well as thier are good Guys everywhere but remember NEVER TRUST THEM! Get it In writing, Get it On tape, tell them your taping your question on the Phone and watch and see how fast they hang up? It's an agency that had to be put under anothers direct supervison after Waco, TX and they are not getting any better! If you go to any Search Engine, Type in ATF Abuses and stand back for the pages you will see! The law Suits settled out of court after thier botched raids and outright non legal activities! Not meaning to turn this post into a "I hate the ATF" deal, but as the old saying goes "Trust, But Verify"? Be carefull out there, it's a jungle and many make thier living on entrapment and criminal breach of our right given to us by the Constitution and God!
If you want to engage in the Life and game of Buying and selling firearms, be warned, the sharks out there are not your friend!
Been there, Done that and No longer have a Licence BY CHOICE!