Can't say about the one you are interested in, BUT if it's of the same quality (I'm sure it is) as the one's at work. The price could be right, very right.
We have, at work, 2 - M500's and one 21".
They are about 30 yrs old, and not abused, and still hold very good on tolerances.
We are a job shop, we're not just making the same old little pieces, day in and day out.
Could bore you with a lot of details, not gonna do it.
If I ever can afford a nice used Harrison lathe, I would buy, for now I'll stick with the 2 I have.
Definitely, check it under power if you can. If it has been used a lot, expect some wear. Hopefully, it has hardened bed ways. If you suspect bed way wear - run carriage up the the chuck end - finger only - lightly snug the carriage lock N back off a hair; now slowly move carriage to tail stock, this will give you an idea of wear. Also, if you can, under power, try it out, run it thru feeding and threading. Pull the end cover (head stock) look at the gears for broken teeth, oil seals leaking, etc.
Good luck.
carl