Even merchants only offer up to 30 days "money back guarantee" on second hand items. I agree - this is a case of tough luck. I know when I buy on Ebay I consider myself lucky even if the goods arrive!
The best you can do is reply in writing (mail it to be sure) using words to the effect that the item was sold in working condition, and in good faith. Whilst you don't doubt the genuinness of the purchaser, you have no way of confirming how the unit has been treated for the past 11 weeks and therefore cannot offer any form of refund or recompense. The item was sold with no warranty, implied or otherwise, and is (unfortunately) now the sole responsibility of the purchaser. I would make sure you use plenty of sympathetic, comiserating language to help soften the blow - maybe recount a similar situation you might have experienced. Bottom line is, keep it light and friendly but tell it like it is.
This will be dissappointing and no doubt annoying to the bloke, but there is nothing else you can do. If there is any bad Karma as result of this, then just be on your guard.
