you certainly have no legal responsibility to refund his money (private sale, buyer beware). i think moraly you're pretty safe too. the fish thrived in your tank and there was no reason for you to think it would do otherwise when sold. as you said, you have no control once it leaves your house. the reality is sometimes fish die. it could be that he didn't acclimatise it correctly, he may have had poor water quality, it may just have been bad luck. whatever the reason, unless he can put forward a compelling argument that it was your fault, it's on him. shame though, it was a nice fish.