It helps to identify type of car. The less picky you are, the better this all works.
What I've typically done is identify something like:
-small engine, high reliability, good MPG, clean.
For cars, 4-10 years old, you can set a price, say $2500. This will put you in a reasonable age range, as that price is low enough to filter out returns, newer vehicles. Set a minimum of $2k. Double all of that for nicer cars that aren't just runners.
Hit craigslist, and friends to find some cars. Research a few of those, and you will center in on one you like.
Then drill down on that car, looking for plausible deals. Call 'em, get their reasons, go visit, talk to them to judge who you are buying from, see the car, and go from there.
It's perfectly ordinary to get a car inspected. You can arrange that ahead of time with your mechanic. Figure out a deposit, or whatever makes sense.
Got a friend who can test drive? That helps too. There are a number of things I do on a test that helps filter out problems. I'll put a couple here:
1. Start the car and let it idle for a long time, until it comes up to operating temp. This takes 10-15 minutes. If it acts irregular, dies, begins to run rough, smokes, it's probably got some issues. Continue, until the fan comes on.
While it's idling, lift the hood, examine things. If it sounds funny or odd, it is. If you see leaks, there are. Simple common sense things here.
2. Get in the car, blast the heat. It should be right there, hot.
3. Blast the AC, if it has it. Should be there, cool in about a minute.
4. With a friend, try everything out. Lights, features, radio, whatever. Test all the electric options.
5. Time to drive the car. Take it out on the road and drive normally for a bit. It should perform like a reasonable car.
6. Find a long stretch of road, put the car in second gear, then drive about 30 MPH for a good mile or two. This will stress the engine, and doing this will filter out cooling system issues, and the engine should be smooth.
7. Exercise the car. Speed up, and use the brakes hard. If it's got ABS, that system should trigger.
8. Take some corners, handle the car a little. Make sure you go fast enough, say 70 MPH, and it should not shake or shimmy.
