HELP!!!!! Micra gearbox problem?

I have bought a Micra s 998 x reg.
The gear stick doesn't return to the central position by itself when in neutral. If you pushed it to the left or right it would stay there. It also feels "mushy" when I move the gear stick from side to side.
I know I have been ripped off by the seller but what is wrong with the gearbox and how easy/expensive is it to fix.