If you can figure out the value of the car you are selling you can profit out of that. Yes it does help if you know which cars people are looking for, but for that matter any car will sell for some price - important you should the price. Firstly find the place where you can find the car, driver around and check including people's yards. If your glance can calculate and tell the cost of the car, the rest falls in place automatically.
Besides from people's yards, tap the classified ads too, you should remember that asking prices are not the sales prices. Help from the car-pricing guides such as blue book might show higher prices. Assumption should be made that the sale of the car is at loan value.
Know nothing about cars?
If you like many others don't know much about cars, but still want to make money from it, find a friend. A deal with him will fetch good returns for you both. All you need to do is fund some dollars to buy old cars your friend recommend and go for the kill. If your friend is also a mechanic or at least good at some repair, you might end up making good profit. Buying old un-used cars, repairing them and selling them is a good preposition as well.
In US different states have different laws, in some, you need to have a dealer license if sell six cars a year. A license on your friend's name, wife name and your name might well make a successful auto business entrepreneur in no time. Check the laws in your state.
All you need posses is knowledge, if you don't have it, let your friend transfer it to you
