How deep one digs into a car is dependent on how much the car costs relative to the market.

If someone is asking a premium price, the car gets a through going over. If the car is a "motor runs rough and needs work" I consider the motor bad and deduct the cost of replacement from market value.

I consider old cars with high / suspect miles are considered to have failing engines / transmissions / worn parts, if those systems turn out to be good, that is a bonus. Don't consider old cars a commodity like buying a 4 year old _fill in the blank_ where the chances of buying a good one sight unseen are good.

I've been in and around the auto repair business for over 40 years. If someone is asking for a compression test on a $ 1,000 car I'm selling , I call it a no sale because the low end buyers are typically the biggest whiners as they expect a 100% new car for no money.

If the car was selling for $ 10,000 and was absolute top of market, I'd do a compression test while they were there ( with a non refundable deposit ). For the most part, the higher end buyer is more accepting and realistic on what a used car is and isn't.

There are exceptions on either end of the scale but that is how things generally track.