I sell a lot of items on here, RC and R2R. And on eBay, I am selling on eBay for around 12-15 years already. Actually if you look at my posts, probably 90% are sell/buy as I don't have time to start my build thread, and I have a lot of equipment to get rid of as I got too many things during my setup.
With anything used, I specifically ask the buyer to test it as soon as they receive it. Package delivered, they test it, it's all on them. You have no control over what goes on for the next two weeks, how the load is on the equipment, heat, moving equipment around the tank, dropping it, power surges, etc, so there's no way you can guarantee it will work for X amount of days after delivery or provide any warranty. Also that's the trick with used equipment - it can break. You pay less as you are assuming the risk, and sometimes things break.
I think you are on the right track. Make sure your bases are covered, as if the payment is over paypal for goods purchased (not gift) he can always open a paypal case and say you sold something broken. The e-mails you have should cover your in that case.