I would buy a 2nd hand gearbox, get your mechanic to look at it first to check seals, general condition and that it is changing, would estimate the cost at no more than €150- €180., might be an idea to change the clutch at the same time.
I did a clutch and 2nd hand gearbox on my sons Fiat recently, total cost including labour was €300, car is perfect again.
Can't see any comeback going back to private seller but you should anyway, you may get some goodwill payment.