You could fix it yourself for nothing - depending on the type of jack socket, sometimes the socket part that should touch the tip of the plug gets slightly bent and this can lead to intermittent connections, I know I fixed this little problem on one of mine a few weeks back.
Literally remove the control plate and look at the connection. plug a lead in and see if it is a good tight connection. If there is any movement or gap between the plug and any part of the socket, this is your problem. Remove the plug and bend the connector so it is now a tight connection. Reassemble and try it.
Anyway good luck with the sale, it shouldn't hang around too long at that price.