Most sellers are honest, but they make money only by buying low and selling high. To survive against a good dealer you must have a value in mind and be determined to buy only if the price is right and the car is sound. Don't assume that the seller really knows the exact condition of the car, regardless of what they say. A part may break after you buy that gave no notice to the seller beforehand. Have the car checked out by someone who knows what to look for.
Compression tells about the condition of the engine.
Look under the fenders and inside the hood for signs of damage repair. Check the mileage against the age of the car to see if its appropriate. Take an extended test drive, including a freeway. Do all this checking before you buy. If you buy and later find out that there were major undiscovered problems with the car, you probably can only recover damages if you show that the condition was known to the seller and hidden.