Since you sell lots of machines for parts, you might have a display port adapter hanging around? Have you tried booting the machine with it connected to an external display?
What we're testing is if there is a loose connection to the display itself, or perhaps broken wiring from lots of open/closing of the laptop.
It still sounds like the machine's graphics processor is pooched, which I believe is part of the logic board?? (don't quote me on that one though...)