The purchaser will always "rat you out." They have no interest in your well being and have no reason to have any loyalty to you. The purchasers attitude would probably be you shouldn't have sold it to me in the first place.
You are not making any sense with this. If I sell a gun and the purchaser uses it in a felony they (LEO) can not trace it to you un-less the purchaser rats you out or you bought it new and even then they most times do not track it. Un-less something really crazy happens the seller is most likely not involved.
Again...not saying it is safe/proper to sell to a felon or any questionable person. If you do a FTF and he looks like the theater shooter (obviously a troubled individual) you can change your mind and I have. Beyond that it is a private sale. JMHO. Do as you will. If that worried put your guns in a LGS on consignment and let them deal with the public. Simplest answer to the paranoid sellers fears.