I hope he gets his price!
My 980 is at 4628.8 after my flight yesterday, so his at 4596 is low, but unlikely to be the lowest. I would be surprised if the N51DM isn't lower time.
That said, it looks like a great airplane, nice equipment, decent US-based history, that is all good.
However, I would point out that the engines are quite high time-- only 800 hours to overhaul. Depending on when the last hot sections were due, you might be able to run it a few hundred hours over (I'm currently planning to do that), but that's not a good look for a very expensive asking price.