When, I occasionally sell off some of my surplus equipment I have a bill of sale that lists out terms, all ready to go, stored on my computer.
I fill it out, print it up and we both sign it.
Not only is it a good thing to do, legally speaking, it just makes sense. It's good service and it shows the other guy that I'm trying to conduct honest, straightforward business.
Just a funny story...
I was working in the theater last Tuesday when a grandmother and her grandkids came to the concession stand and ordered popcorn and sodas.
I thought I heard them order "Coke" but they wanted DIET Coke instead. I dished up the corn and one of the Coke's but they caught me and corrected my mistake.
I said, "Oops! Bartender's Mistake!" I gave them their snacks and their drinks plus I gave them the mistaken drink for free.
Grandma didn't want to take the drink and asked, "Are you sure?" I told her that the health code says I can't sell it to anybody else so I'd have to pour it down the drain.
She was happy to have the drink and she ended up buying a box of candy, to boot.
Playing it straight often has its benefits.