I have no idea what the item you are asking about is but I am rather disturbed that you consider the feedback issue to be the reason you would base your decision on. It would be the last thing I would consider, I would base what I did on the level of customer service that I hope a seller would extend to me were the roles reversed.

I would probably offer a partial refund if there was any way to repair it, a full refund if they sent pics and it couldn't be repaired. If that was not acceptable to you or the buyer I would ask them to return it and say I would have the item tested and if it is a manufacturing fault I would refund including cost of return.