You will need to get a thorough pre-purchase inspection by an independent mechanic. This should be a general examination.
Test drive the vehicle before purchase, Online specs and descriptions are not a replacement for a test-drive.
Know your trade-in value.
Check all documents properly and ensure authenticity before payment is made.
Make sure you really like the car you're buying and that it really meets your needs.
If the seller will not make the vehicle available for a pre-purchase inspection by a mechanic of your choosing, you should not buy the car.