Did you sell the game to someone locally where you can physically drive to that persons house to verify that it's not working, or that you can fix it ?
When you transport a game weather it's done via a professional shipping company or tossing it into an SUV sometimes things become loose. I would see what this persons knowledge is on trying to walk them through.
Maybe a chip just needs to be re-seated, possibly a molex connector came loose. It's probably something simple especially if you said it was verified working before leaving your place. Did you mark the PCB serial number to make sure he's not trying to swap in a bad board purposely ? Does KI1 AND KI2 Use the same pinout ? if so have him switch the boards and see if it's a board issue... than the power supply... than monitors. It's usually A, B, or C.