Are you a dealer?
Did you say anything about the condition of the gearbox at the time of sale?
And I presume that all of that is true?
On the face of it then he does not have a claim. The only way he will be able to claim is to argue that you are a commercial seller. If you are a private seller that you are not bounds by the sale of goods act or consumer rights act as it now is.
The only way he could argue that there is a claim is to full-back on the common law and argue that it is implicit in any contracts that services rendered or goods supplied will be of a reasonable standards but there is quite a lot of case law to the effect that a person who buys a second-hand car does so at his peril.
Can I clarify anything for you?