Confused about what the other guy said, why would he need to go to a skirmish site to test the gun - the only thing you can't test in an ordinary room is how effective the hop up is, and anyone buying an airsoft gun should have the basic knowledge that the hopup/bucking/barrel are conveniently isolated from the complicated parts.
It's rather naive to not test a product because of ignorance - ignorance of the fact that you don't need to be at a skirmish site to fire the gun - or even outdoors.
The buyer went to a game day - gun stop working after 20 mins, went to another game day (thinking it would magically fix it self if left unused in between?) then decided to return it?
Could have some sympathy if the guy is genuine, but he should have got back in touch as soon as the problem was detected, not after he tried to fix it - and then took it to two "professionals".
On the basis that he put it in a techs hands I would decline to take it back, you've no idea what a "professional" might have done, let alone the owner. As for negative feedback, can you reply, if so, I think that the buyer did not test the gun until a skirmish says it all really, you've no idea what he did with it until then, you've no idea if it broke in shipping.