As a private seller, the only legal points that must be complied with are that the seller has the right to sell the car, that it matches the description provided by the seller and that the car must be roadworthy. Assuming that the buyer inspected the vehicle themselves, took it for a test drive and were happy to hand over payment, then the seller has complied with the law. In order for any court action to be successful the buyer would need to prove that the seller was aware of issue with the gearbox before the sale. You may wish to seek independent legal advice in the event that legal action is taken by the seller.