You can always try to sue. Your chances of winning are slim to none if the sale was as-is and you have no proof to counter that. Sorry and good luck.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
In Colorado it is assumed that the sale of a used vehicle is "as-is" unless the seller makes warranties or representations about the vehicle's condition. You will have to show that some promise was made about the condition of the transmission or you will be out of luck. You should have a mechanic inspect any used car that you are considering purchasing.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.
I have actually taken a similar cases to this to trial. The primary concern the judge had was whether the seller of the car knew about any defect to the car prior to its sale. If the owner knew about a problem with the car and and misled the purchaser, then there could be a form of misrepresentation. In such event, in Colorado, the purchaser has two choices in a remedy. One, he could sue for rescission which is an unwinding of the contract. This means he gets his money back, and seller gets the car back. Alternatively, the buyer could sue for damages. I that instance, the purchaser would be entitled to the value of the car "as represented" minus the value of the car "in actuality". So, if the seller represented the car to be valued at $10,000 but it was only worth $5,000, the buyer would argue $5,000 in damages.
Selling a car 'as is' is only a defense to a breach of warranty claim. It does not protect a seller from misrepresentation claims.
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