just send him the 40 fucking dollars. good christ. why does everything have to be a conspiracy with these types of deals? You saw his feedback, thats a pretty damn good indication of the kind of trader he is.
You even said it yourself, you did not thoroughly test this system before putting it up for sale. I'm going to assume here that the system was used when you took ownership of it, and then turned around and sold it to someone else.
Who knows how long that lens could have been going out?
Take ownership of your sale, and do the right thing here and send him the 40 bucks.
Even if he does dick you on it, at least you stood up and did your part through to the end.
/rant