Over the years, I've bought 40+ fully transferable machineguns from private individuals to big volume dealers. Here are my experiences:
1) Most sellers want 100% of the funds up front - but some will do 50% up front and then the other 50% when ATF approval is obtained.
2) Written acknowledgements are usually done by email (and less frequently text). Some sellers will telephone, but I usually ask for a confirmatory email because that gets the deal "in writing". Many sellers will not send a receipt unless asked to do so.
3) Guarantee of transferability - vet the seller extensively. Ask for proof of life photos and references. Don't send any money until you are satisfied that the seller is trustworthy and legitimate.
4) Penalty for backing out of a sale - I've heard of it - but good luck with that. Just get your money back and look elsewhere. Heck, if you are that worried about the seller/buyer then don't even consider doing business with them in the first place.
5) Most deals are done on a verbal agreement between buyer and seller.
6) Buying a machinegun from a private individual unknown to you can be risky but very good deals can sometimes be had.
7) Buying a machinegun from a dealer (especially a well known dealer) is much less risky because they depend so much on their reputation for their continued business. However, the price will likely be higher.
8) I'm not sure what you mean by notification of seller's heirs of pending sale. Unless, you mean that the seller dies before they ship the gun to you or your NFA dealer. I have never had this happen but I suppose it could be a problem if the decedent were a sole proprietor dealer or private individual. However, machine gun sales are a private matter and I believe that most people would prefer to be discreet and not tell anyone else about the deal.
9) For really rare/high dollar machineguns, I fly in to inspect the firearm before sending any money. If the seller declines to meet face to face, walk away from the deal.
10) Finally, if it seems to good to be true -- such as a rare or very high dollar item priced for quick sale way below fair market value -- it usually is.
I hope this helps.