Treat it like a private sale from a mates mate who you know is dodgy.
Came in with 110k now has 107k, might have done another 50k or more in the mean time.....
Expect no warranty or guarantee or follow up support.
Check the car out, have it thoroughly inspected by your chosen mechanic.
If your still happy with the negotiated price, buy it and be happy, and deal with any problems that arise like you would if it was a private sale.