Apply for FFL class 1 license, then pay an annual Special Occupational Tax of $500, plus meet all local and state regulations calling for any permits or licenses. Then, AFTER you receive your FFL and tax stamp FOR THE LICENSE, you start the transfer process to transfer each individual firearm to you. There will be a $200 tax on each and every firearm transferred from a private individual or estate to you, the dealer.
Once you have the approved transfers back from BATFE, you may take possession of the firearms. At that point you can advertise them, sell them, or trade them. If you sell to a private individual, there is another $200 transfer tax stamp required for each firearm. Those transfers go on a Form 4. If you transfer them to another SOT dealer, whether in your state or another state, there is no tax due on those transfers. Those go on a Form 3. These days, typical transfer approval times are running 4-6 months for each transfer. Good luck.