If it is legally in the NFA registry you will need to complete a form 4 (either as an individual or trist/llc). When it is approved you may lawfully take possession of it.
If its not in the registry.... well...not legally happening.
If it is legal make sure you are getting it for a steal. They are of a problematic design and have had some issues. Could be BS but do your homework. Keep in mind that you will be paying at least $200 and will be stuck with it until you can find a buyer.
What is your attraction to it? There are certainly many newer updated less expensive Non-nfa guns on the market today. IE the Molot Vepr 12ga.