This is just my opinion but I'd go for a private sale rather than from a dealer. When trading in to a dealer you usually can't get as much for the car as if you sold it yourself so a lot of people will trade in their "problem children" cars just to get rid of them rather than try to sell them on their own. They're willing to take less money just to get it off their hands.
The exception to this can be when the dealer offers a warranty on the used car. If the dealer is willing to put a 2 or 3 year warranty on it then there is a better chance that the thing isn't junk. You will probably pay more than you would in a private sale because of the dealer markup (they "buy low, sell high", no surprise there).
Have someone who knows cars well go with you to test drive them. Check all the oil and fluids before you start it up - see what color the oil is and if everything has been filled correctly. Don't let them warm it up for you before you get there. I won't buy any vehicle that the owner "warmed up for me" because I want to see it start and want to check all the stuff before I do that.
Run your finger around the tailpipe - is it sooty? Greasy? The exhaust and tailpipe mung can tell you a lot about the engine.
Ask the owner if he/she has the maintenance records on the car. Good owners will keep a log or all the receipts of when the car was serviced. I always save mine because it's a good selling point if I ever want to sell the car.
Review them thoroughly before making an offer.
Test drive it then have your knowledgeable partner drive it too. Then bring it back, let it sit, then check the fluids again, engine, the ground underneath where it was sitting, the color of the exhaust (blue means it's burning oil, black can mean a lot of different things but could be running rich because of bad maintenance or carbon buildup, white that smells like burning sugar is a very bad thing because your head gasket could be going), etc.
Best yet, have it checked over by a reputable mechanic before you buy it. This isn't always possible but do it if you can. If the owner severely balks at it you may want to walk away - there may be something bad you would discover if you do that.
Never fall in love with a car, there will always be another one out there to look at so if anything is fishy with it walk away.
It's still a crapshoot but you can minimize your risk by checking it out thoroughly. The younger the car is the better chance that it hasn't been trashed yet.
Always ask the owner why they are getting rid of the car. If someone is selling what looks like a perfectly good minivan that is 2 or 3 years old only to buy another minivan it makes me wonder if the one he's getting rid of was a lemon and he's had problems with it. A lot of times you can get good small cars from families who just added another child and now have to trade up to a minivan because they need the room.
Good luck, I hope that helps (others here can probably add alot more),
LQ, eternally grateful that my father was a gearhead
Thank you. Thank you. Thank you.
I've learned more from reading your post than I could have learned from books and magazines.
I'll save this post for when the moment comes.