What does it mean for the reverse gear to grind and pop out of gear on 2002 mini on 2002 Mini Cooper

I'm thinking of buying this car, but the owner says it grinds in reverse and then pops out of gear sometimes. What is this & what kind of repair cost and I looking at? Car has 123,000 miles on it.

Asked by for the 2002 Mini Cooper
its either needs a transmission or shifter cables so price could be from hundereds to thousands. have it diagnosed before you buy