I haven't sold many machines (3 in total), but I have always been 100% honest from the very beginning with detailing the goods and bads of a machine. I then encourage them to have a few games on pickup but once they take the machine and it goes on it's bumpy ride some where to it's new home then it's up to the new owner - just the nature of a static machine going for a bumpy ride. I did have to offer some advice / help once but unless your selling a new machine or a high end restore I think it really falls on to the new owner.

I bought a machine from a dealer in Qld once and it had a few problems on power up, in which I didn't demand free fixing but was hoping for advice - my luck was that I got both but wasn't expecting both.

So I reckon good advice is invaluable, just state everything up front.