eBay/PayPal advice on return request

Ok, here is the situation. I sold an ASUS motherboard (A8N-E) which was barely used over a one month period. It was sold with all of the original accessories and packaging with almost 3 years left on the warranty. It worked great when I had it.

The gentleman who bought it had to have it quickly. He bought it early on a Saturday morning so I did something I usually don't do and rushed to the Post Office to send out the package via Priority Mail. He got the package on Monday. I took a small hit on the shipping. I charged $10 and it cost me $10.84 in shipping fees (not counting time materials or anything like that). He left me a positive feedback immediatly.

Well, over the weekend I got a couple of emails from him. The first said that the B1 memory slot didn't work and that he could only get it to boot up 2 out of 10 times. He told me that he wants to return it although he said if I insist on not giving him a full refund, he wants me to help him RMA it with ASUS. I sent him an email saying that I would help him RMA the board. I did this because a) it worked for me and b) because I had also sold my only socket 939 processor so if I got it back from him, I couldn't resell it because I didn't know if it was broken or not. He sent me another email saying he had already bought a new one from Newegg and he doesn't need a replacement from ASUS.

Ok, so as I see it, there are three possibilities. One, it is indeed broken. Two, he is using components that do not work with the board. Three, he just doesn't want the board anymore and wants a refund.

What options do I have? If I say the board works and he doesn't, who will PayPal side with. I have always heard that PayPal will side with the buyer. Is that true? I am looking for advice from anyone who has experience handling returns through PayPal. I have processed a few but I just gave the full refunds and took the hit. I do not want to do this here. I bent over backwards for this guy to try to make him happy. What are my options?