If I am the seller you pay the money before the transfer starts. Depending on if you know or how well you know the person, you work out how much. I would never start a transfer (sale) with out 50% down and would only do 50% on high end items, and that is rare. Mostly for people I know. I would also let you know (spelled out in writing and signed by the buyer) that if you fail to pay the remaining 50% when the transfer is approved, it will cost you XX%. Somewhere between 10-20% of your initial payment. When I say high end items $30K+.
Also, if the deal is a 50/50% sale the seller should specify that you have x business days to send funds after the transfer is approved. This is negotiable but normally 10 business days should be enough. I am not waiting 6 months for funds and ATF requires you complete the transfer in a reasonable time frame. I don't remember if they list days but I wouldn't want a "approved F4" item in my inventory at a compliance inspection that had been sitting for 6 months. It happens, but not often due to deaths.
I have heard too many times, well I don't have the cash now, wife found out, wrecked the car and have to fix it, kid needs braces, etc. When in reality you changed your mind after 10-14 months. Too bad, you bought it and now can't/won't pay?
If you pay 100% I am willing to give you all the "parts" not registered that can easily be taken off the item. IE M16 - I would ship the upper and accessories (if any) on your dime, MP5 - bolt, trigger pack (if reg rec, not if sear gun), accessories, on your dime. Nothing that would risk any dings or damage to the item.
If that sounds harsh, too bad. Like I said, too many times someone bought a NFA item and after a long wait time couldn't or wouldn't pay. Also, be real sure you know you can clear the ATF/NFA/FBI background check. I have seen old issues pop up that were forgotten about or they thought that "thing" went away somehow. It happens. It may require a lawyer to have it cleared up or even more. Think $$, $$$, $$$$ and more $$$$$$. This also adds to the transfer time frame.
Transfer Info
Same state:
Form 4 from seller to you $200 tax 10-14+ months for the approval
Estate form 5 (if you are in the will) no tax, if not Form 4 $200 tax same long time frame. Copies of will, etc required with forms.
Different states:
Form 4 to a dealer in buyers state $200 tax 2-10 months, Form 4 from dealer to you $200 tax, 10-14 months+ for approval
Estate - need to double check this so won't comment, different if you are in the will vs not. If not it will transfer just like a regular seller/buyer out of state but requires copies of will, etc with forms.
If estate, you better hope the executor knows something about NFA items or they are smart enough to ask a RIK for help. Veteran estates can be the worst as some items were bought home on approved military paperwork and items are legal to own and transfer. A lot of NFA has been lost because the paperwork can't be found and no one has copies.
