It's all about the brand and previous owners. Personally, I only buy used from Honda and Toyota. They have a reputation to uphold for reliability. Buy a carfax. $20 well spent to know the history of ownership. It's not foolproof, but generally good enough. Check with your friend group and extended relationships to see if they are selling cars. If they know you, even through a friend, hopefully they have enough shame to not try and screw you on a lemon.