I think its this part where the problem is "but said he couldnt afford it".

He's bought something he couldn't afford but liked the look of, who's bought something with their heart not head before?? Then, it doesn't work with his set up so, now he's upset that he's spent money (he couldn't afford to spend) on something which he can't use. Its a bit like losing your wallet! I've been here before.

If you knew it wouldn't fit his set up and willingly let him buy it knowing it wouldn't fit and thinking he was a plank then I think you're bang out of order, you took advantage!

It you didn't know this and are confident you can sell it easily enough then why not show some goodwill and give him his money back and sell it again. He bought your radiator fan off you anyway so its not like he wasted your time completely.

Its your call isn't it but I think what goes around comes around. I'd sooner sleep better knowing you were the bigger man ultimately.

My 2p worth.