you are buying a used car with over $100k miles, not a new car from a servicing dealership. I think who you are buying from is almost irrelevant unless they are providing some sort of warranty .
A good question to ask would be: What are you worried about? And would it not be the same worries regardless of who you buy it from (dealer or private party)?
it's up to you to protect yourself and due the normal due diligence, as previous poster mentioned. Like others said, have it checked out by a 3rd party experienced mechanic/tech.
Good luck.