I purchased a car from a private owner off Craigslist. The next day I noticed a knock in the engine. My mechanic informed me that I needed a new engine. Can I sue this person for the engine and labor?
In a private sale where you buy "as is" is are stuck with what you get. See www.ConsumerLawyerHwlp.com for a discussion of auto fraud.
Generally the answer is no since most cars are sold "as-is" with no warranty. If however the person made promises to you regarding the condition of the car which were false you may have some recourse.