Have you checked the fuse in the fusebox behind the glovebox to make sure it is not blown as well?

As said above, locking modules can go tits up for the fun of it, plus also, if one of the front locking motors plays up the central locking can do all sorts of weird things, so the easiest thing to do if selling is to take the fuse out (or replace with a blown one in case anyone just looks quickly) and say it is something easier (like a key needing programming).