Thank you for your reply. It's frustrating to know that by paying for the repair, I'm admitting fault.
I felt like repairing the issue was the right / honorable thing to do
Well therein lies your problem. You have clearly stated that fixing the issue was the right/honorable thing to do. If the problem still exists and you no longer want to fix the problem, what does that make your action? Hint: it's the opposite of right and honorable.
If your position from the start was: it's not my fault or caveat emptor, then you would have more of a case. But when you offered to fix the problem, you also offered to
solve the problem to completion and accept liability. You can't just walk away from that once it becomes too expensive or too much of a hassle.
In other words, say you saw a guy stranded on the street with broken down car. You try to be the "nice guy" and tell him not to call a tow company or AAA and start to help by poking around his engine. Eventually, you get a call and have to leave or realize you don't know what is wrong. Well even though you were trying to be the "nice guy" you didn't help and if anything made things worse because you delayed him actually calling a tow company or AAA (which may be closed now) or you may have fiddled or changed things in his engine that he, or the repairman might be unaware of. The point being, if you weren't going to be able to help the person, then you probably shouldn't have helped in the first place. (Which reminds me, this is not legal advice and I'm not trying to help you. :wink

This often comes up in CPR cases where a person who is not qualified to give CPR (or just doesn't do it correctly: broken ribs etc.), administers CPR to an unconscious victim. The person eventually dies, but it sometimes is not clear if the person did more damage by administering CPR. Even though that person was only trying to help, they could be partially liable for the person's death.
Good intentions mean nothing in court. You could say you were trying to help a guy when you stole his laptop because you thought he was too stressed out and needed to get away from work for a while. Actions mean everything and when you committed to helping solve a problem, you are then on the hook for completing that action. Now the buyer might be pushing a bit too far in the other direction by asking for a full refund plus legal fees (I'm not sure on what basis he can claim that since you do not have a return policy) however, if I were you, I would be prepared to finish completing the repair work to resolve whatever issue is causing the power loss. When you took it upon yourself to help, you delayed or deferred the buyers ability to fix the problem himself (like telling the stranded guy not to call the tow company or AAA in the example above.) He can argue that you restricted his ability to use his own trusted mechanic who would have fixed/diagnosed the problem correctly when you took control of the situation and took it to your repair shop.