You will rarely be able to buy a car from a dealer and flip it for a profit, as these cars are generally sold at their highest price. Random people, however, don't have to pay employees or rent on a lot, and can afford to let go of their used car for much less than a dealer can. Confine your search to these sellers for the best results.  Don't be immediately turned off by "salvage titles," which are often some of the best deals on the market. Be sure, however, to ask what happened to earn the title. A broken window is a lot different than a cracked axle, but both could cause the car to be considered salvage.  Carfax and other title check sites are good ways to get a cheap vehicle history for bigger buys. Places like Edmunds and Kelly Blue Book let you input the car's make, model, year, and condition and see the general price. This is often a good baseline for negotiation, and can be used to effectively bargain if someone is grossly overestimating the price. These sites usually have ratings too, letting you see if the car has common issues to look for or has a record of reliability. Some tips to make the most of these sites include:  Note the "dealer's price." You can usually bargain down to the price they expect from a dealership, because there will be much less paperwork for both of you. Check several sites at once, and always assume the car will be in worse shape than it is -- most people will try and sell at the highest price they can find online, and will err towards a better condition on their advertisement. Be wary of any cars you come across that are already on and running. It takes the most energy and momentum to get a cold car started, and you can often hear or feel any issues with a bad motor as it lugs to a start. Be on the lookout for:  Cars that don't start easily or smoothly. Loud noises or grinding sounds in the engine. Ticking, whining, or shaking as the car starts. As you do, ask about the car's history, starting with its last oil change. The lighter the oil, the better, and it shouldn't ever be dark brown or sludge-like, especially if it has been changed in the past six months. If you see any water or solid bits (like metal) in the oil, walk away -- repairs won't be cheap. Have the seller rev the engine 5-6 times and watch the exhaust. If produces black or thick smoke you should walk away, especially if the oil levels are also low. A little water is to be expected. But oil or radiator fluid is a big no-no, and the car likely shouldn't be bought. Another good way to check this is to have someone rev the engine with the radiator cap off. If you see air bubbles in the radiator when the car revs, walk off -- the head gasket is likely blown. Is the engine temperature high? If so, and there is hot water leaking from the car (not fluid or oil!), then you might be in for a bargain. This could just be a leaky house causing the overheating, meaning you can cheaply fix an expensive-seeming problem. There shouldn't be loud ticking noises or gurgling, and it's pretty obvious when something doesn't sound quite right. Have someone rev the engine in neutral and watch to make sure it still sounds good as you watch for any issues. Turn the car off and check the belts and hoses, looking for corrosion, rust, or heavy wear. While 1-2 belts and hoses are easy to fix, a complete overhaul could cut away any profits you hoped to make. If you're a true bargain hunter with some mechanical skills, you might buy a non-working car cheaply if you believe you can get it running. But in all other cases, you want to take the car for a spin. Test a variety of speeds and variations, ideally testing it in both a neighborhood and on a highway or road where you can get the speed up safely. As you do, pay careful attention to:   Steering: is it smooth and responsive?  Braking: Does it stop the car quickly, especially in quick-stop situations? Most importantly, does the car stop in a straight line?  Transmission: Does it shift smoothly? On automatic cars, you should be able to get into drive and reverse in less than a second and a half. Any longer delay could be a bad sign.  Electronics and Features: Do the lights, windows, and AC work? Is the odometer still running or is it stuck (and do they know how long ago it stopped running, if so?). This is especially crucial if they didn't mention these deficits in the ad posting. Most people are honest and upfront about issues the car might have, but you should be constantly searching for other issues to use as a negotiating tool. If there are minor to serious issues that they didn't tell you, but you're confident can be fixed cheaply, you can often get serious discounts.  As you're examining the car, show them issues as you find them. Most people trust a calm, thorough buyer if you walk them through your tests and issues, making them more likely to take your word on price. Gauge their own car knowledge as well. If they seem lost around an engine, you can use this to your advantage to play up any perceived issues. If they don't have a title on hand, you should be very wary about buying the car. The title is required to get the car registered, and to subsequently get insurance for the vehicle. If they can't hand over the title, you better be sure the car is worth the hassle of getting it re-titled. For more expensive cars, use a site like CarFax to get the title history and avoid any unpleasant surprises.

Summary:
Search car auctions, classifieds, eBay, and Craigslist for cars sold by their owner. Use online appraisal sites to get a good idea of the base price. Always start the car from a cold engine. Check the oil levels and oil color. Search under the car for leaks once it's been running a while. Pop the hood to listen and look at the engine. Drive the car before buying whenever possible. Use any and all perceived defects to drive the price down. Be sure to get the title and copy of the car's registration.