When I sold my work I wrote up contracts for buyer and seller.
I wrote exactly what was being sold and for how much. I also added that as a seller I relinquished all responsibility for the work and the buyer assumes all responsibility.
Both contracts were signed by and in front of both parties and a witness who also signed them if I remember correctly.
The customer lists (books) were handed over at the same time in exchange for full payment which was transferred into my bank account.
I sold my runs off in 3 stages, all to the same buyer, and separate contracts were drawn up for each transaction and the same procedures followed.
Everything went smoothly on every occasion and there were never any disputes or problems.
I may still have my copies of the contracts somewhere so if I find them I'll post them up.
John