From what you are describing there may have been a lack of detailed analysis but it is hard to say if there was obvious negligence. OTOH, if you think there was negligence, then pursue the seller/mechanic in small claims court or contact the attorney general in the state the car was sold. IMO you’ll be in for less grief to spend the money on fixes or to sell the car and put it behind you if you’re unhappy.

Some people’s expectations are reasonable in their view, but ultimately out of line for a typical used car. Plus, there is the sight unseen part. That guarantees surprises because no one will look as closely as a buyer with an experienced eye for details.