Hi Guilherme,
You can't just return something because it has two many defects. Obviously its a second hand car so you could expect a level of degradation from new. So be clear about what your issue is with the car. The seller is bound by common law contract requirements. Which means they can be liable for misrepresentations that you the buyer relied upon.
If the seller assured you that a specific item was in excellent condition with no issues, such as the gear box but you find that the gear box is defective and needs to be replaced the you may have grounds to take legal action to rescind the contract.
Please keep in mind the grounds to rescind the contract may be if there is one or more major defects. I would grab a copy of the ad if its available and see if there are any false statements in it. You may want to speak with a lawyer to draft a letter to the seller.
This blog post might help:
Faulty Second Hand Car? Your Options as a Buyer - Legal Blog - LawAnswers.com.au
