It should be fine. I had a guy that sold me something off ebay saying it was a working pull, as is. It was DOA. I negged the guy, he negged back in revenge even though I paid on the spot, etc. I told him he can't sell DOA crap and hide behind as-is. He refunded me, we withdrew the feedback.
I sell stuff on there as-is too. It's all working, etc, when I sell it. I describe it exactly. I just don't want them blowing it up or changing their mind, then pawning it back off on me. In computers it happens a lot.
He's explained that it works fine. It probably does. He's changed the cooler on it, so it's hard to say whether the old one seized on him, he was overclocking it, he didn't like the look of the old heatsink, or whatever. He could have also damaged the chip when he changed it. There are variables, but just make sure you take a screenshot of the auction, then if it doesn't work when you get it you try and work it out with him. You can always neg him and file a Paypal dispute in a worst case scenario.