As one who currently drives a used car (used when I bought it) here are some pointers I use when shopping for one:
1. Get the VIN number of the car and run a CarFax report on it. (
www.carfax.com) It's free to get the basic information, but if you need more detailed information it's $20 for the service. The CarFax report will tell you the vehicle's registration history; whether it was declared 'salvage' because it was totaled or had a previous use as a taxicab or a rental car. (Contrary to popular belief, not every former rental car is trashed out; some of the higher-end car makes used by rental companies are in pretty decent shape and have had regular periodic maintenance done on them.)
2. My personal preference - get a used car with less than 50,000 miles on the odometer. Some finance companies will not finance used cars unless they have less than 50,000 miles or are less than five years old. Most lease-return cars are well taken care of. Some of these will fall under the 'certified used car' program, where they have been inspected and still carry the remaining new vehicle warranty.
3. Try to get a car that has good availability of parts. Generally, most automakers will have a five-year supply of parts made for each make's model year in their system; more popular makes may have even more.