Is the Trans, Manual or Auto?
If you repair the trans, you should get some use out of it. to recoup the
cost of repair.
If sold as is, will it pass smog with the bad trans. Might be tough to
sell that way.
When was Trans fluid changed out.
Have you tried to drain and refill trans fluid?
Might also try some of those additive for the Trans.
Also take to a different shop and get another opinion.