First off, if he is in the business of buying guns, he is probably an FFL holder. Whatever transactions he does, he will give you a receipt. He will probably ask you to fill out some documentation that he bought it fom you and you may have to sign it. Basically this is what I have run into and it's not a big issue. Whatever he requires is part of his CYA practice ....
Sorry for the curt reply. Yes a sales receipt clearly identifying the gun with serial number and the name of the shop should be enough of a paper trail to protect you.