Gearbox question. 1994 Celica GT

I'm a Newbie on this Forum and have a question.
My car is a 94, Celica GT, manual, totally immaculate, no bumps, no shunts, no sprays and with just under 75k miles.
Problem is that synchro is totally shot when selecting 3rd gear.
I now need to sell this beauty, but the gearbox problem obviously means that it's not a saleable item.
Thoughts...a magic formula gearbox oil...a 2nd-hand replacement box...or somebody to buy it (at a sensible price).
This car is in SUPERB condition.
What do I do?