I feel compelled to put in my two cents even though others have offered good input.
Especially given the prices of machine guns, there is a lot of trust in others required to buy machine guns. The gun will be registered to other parties as the gun gets to you, the final/actual buyer. Those people have the ability to take advantage of their temporary possession of the gun. It doesn't happen a lot, but it can, and it does happen once in a while. Pick your local dealer carefully. Buy from a trustworthy source. Ideally you wouldn't have to pay for the gun until ATF had approved the transfer, so you know the gun exists and is not going to be transferred to anyone else after you paid. But the machine gun market with a very limited supply and plenty of demand lets sellers dictate the terms, so payment up front on trust is the norm.
As others noted, ATF registration is not about ownership, it is about legal possession. Who "owns" it is a function of who paid for it and who has the right to control its destiny. Often the legal owner and possessor are the same, but when an NFA gun is moving to a new owner they are not. Contracts can help here. Being confident that the parties you are doing business with are trustworthy also goes a long way.
One final suggestion. If you are thinking you like or are going to buy more machine guns that are also considered curios or relics (like an Ingram M6) you can eliminate having to use a local dealer buy getting a curio/relic FFL. The holder of a c&r ffl can get the gun direct from an out of state seller, like the auction house you bought the gun from. You still have to do a transfer form, still have to pay the transfer tax (unless the gun is deactivated) but you eliminate having to use a local dealer and trust that he or she is honest and will take care of your gun while they have it.
Edited by Bartlow, 14 January 2021 - 04:44 PM.
