What kind of condition is it in otherwise?
You won't get top price for a real nice truck if it doesn't run and needs a new motor. On the other hand, if it needs much other work you're not going to gain as much replacing the motor as you are likely to spend doing it UNLESS you can do it yourself and get a real cheap or free motor.
My other question is can you trust the diagnosis this mechanic made? These are low compression motors to start with, and they are relatively noisy compared to today's motors. If your mechanic is young and doesn't have much experience with these motors you may have received poor advice.
And to go a step further, some people do not like these early sixes, and may be inclined to condemn one not because its worn out, but because they would rather work on something else.
With those comments made, I went back and reread your question. If you are going to sell it regardless, sell it as is. You won't make money fixing it.
