Does te fact that the 240Z has an automatic diminish its value?

Only a minor amount given the cars are in equal condition.

From what I've seen, not much difference in value, but most 240Z buyers want a manual tranny, so they can be more difficult to sell.

Here's a little different spin on the subject.

If I were looking for a 240 to build I'd almost prefer an automatic car as they are harder to beat on. Going for a 5 speed anyway, a tranny swap would be in order so no big deal.

We see this in the 510 realm all the time. Most stick cars have been beat to death while the automatic cars have been Aunt Edna's grocery getter.