1st: Stop thinking the way you are. You CAN NOT get screwed, when buying a car.
Think about it. You're there, looking at the car, have time to inspect it, test drive it, etc. Then, you make the choice to buy the car, and put the money towards it. You can't get "screwed over" because you're the one making the choice to buy it.
2nd. Do your research. And lots of it. Find a car you like, and before you even talk to the seller, look up EVERYTHING you can about the car. Ask your insurance for a quote, etc.
3rd. Test drive the car, and ask to have a mechanic inspect it. This will be your cost, but is worth it. Ask for service records, etc. Find out how the car has been maintained.
4th. IGNORE MILEAGE!!! Whether a car has 60k, or 600k, neither tells you how the car is actually maintained. A lot of people advertise "low miles" but don't mention that the car was abused for all those miles.
That should about do it....Oh, and don't finance. Ever.
Bought and sold about 20 cars over the last two years.