Make sure when you're buying your used vehicle that not only are you going to keep it well maintained, but that the previous owners also did so. Ask them for maintenance records if they took it to a dealership or if they wrote down when they did it themselves.
A few things you can do yourself to check if they seem sketchy:
Check the oil and wipe the dipstick with a paper towel, it should be brown and, well, oily. If its watery, muddy, milky, or sludgy, stay away!
Try to wiggle the tires to see if they're tight or not. If they're loose, you could be looking at replacing tie rod ends, which aren't that difficult to replace, but its a sign that the car hasn't been maintained.
Get up under the car to look for rust. It may have been painted over or bondo'd. Bubbling paint usually equals rust too.
Make sure when you go look at a car, that you get to start it and hear it cold. Listen for ticks, knocks, and rattles. These are bad. If the owner has "already warmed it up for you", they could be trying to hide noises the car is making.
Lastly, almost every car has its infamous series of issues(Jeep 4 litre flexplate knock, GM 3.1 litre intake manifolds, etc). Research your car thoroughly and be prepared to fix these issues when you buy your car.
Good luck!