In my experience the best used car sellers are typically people who don’t have a hidden agenda (i.e. problems to transfer to you, the buyer). I’ve found they tend to be forthright and pleasant to deal with, and their cars are usually the best.
They simply would like to obtain a better price than a dealer would offer them in trade, though that amount is typically below what a dealer would sell the car at retail for.
These people usually advertise their cars at fair prices, in line with Blue Book values for private party sales. You can tell a lot from the state of their neighborhoods and homes, like how they tend to take care of their cars. Original owners selling well maintained cars with service records tend to live in nicer areas.
Good reasons for selling are because they bought a new car due to family changes (baby on the way or kids left home) or simply because they want to buy a new car. Crucially, they don’t use fast taking, high pressure tactics, and they don’t exhibit squirrelly behavior which is a mark of dishonesty.
Honest sellers are also unlikely to have stories about the car’s recent history that would raise a red flag (e.g. “I bought it off a friend last month who…”). As a buyer you have lots of choices of sellers to deal with. Know how to seek out the good ones.