That definitely doesn't look like an ebony fretboard to me, looks like rosewood... So there's one inconsistency already with what he's telling you (unless my eyes are just bad tonight, since it's 3:00am and I'm still at work). Other than that, I don't know a lot about the old ESP models. It looks right, I once had an early 90's ESP George Lynch Skulls & Snakes guitar, and it had the "Custom Guitars" stamp, but no serial number, if I recall. Unless it was on the neck plate, but I know there was none on the headstock. My current custom, also from the 90's has no serial number or even a stamp on it, so it's entirely possible it could be legit and well worth $500. I'd wait for others to give you better information though.