I sold a computer on Kijiji, and now the buyer is saying the is a problem with the battery. How should I deal with this problem?

I sold a HP Laptop to a Kijiji buyer. The buyer is claiming that there is an issue with the battery. More specifically he says it looks like the battery is missing. The battery will not charge, and the computer turns on but only stays on while plugged in. The computer has been sitting unused for three months until today, but it was working fine last time it was booted up, and when I booted it up today to show the buyer it was in proper working condition. I recommended troubleshooting the issue from advice on the HP website help page. That's as far as I have gotten. Do I have an obligation to give him his money back if that's what he wants? If I say "no returns", is that going to cause me more problems down the line? I am worried that if the problem is not resolved for the buyer, and I don't make a full refund, that I may have legal problems.