Many people recommend buying a used car from owner, and while I don't disagree with it, used car dealers can also be helpful. Just look at their Yelp\Google ratings and maybe better business burea to see if there is no major unresolved complaints against them. If there are, you should probably question it.
Check out the Carfax, blue book value, and see even if you can take a mechanic with you to check under the hood.
Goodluck