And the Top Three causes are :

#3 - ignition switch going out
#2 - starter solenoid getting bad

...and the number one cause: battery cable connections need cleaning.

The latter usually gets very bad within a few days and won't start at all.

Troubleshoot like this to avoid buying un-needed parts:
1. Clean the battery terminals and cable ends. Fasten securely

2. If that doesn't resolve the problem, test the ignition switch by bypassing it:
a. Put car in nuetral and block the wheels
b. Using a remote start cable, connect the starter "S" terminal to the battery +. Press the remote start button to test.
- It it cranks consistently this way, but not when using the key, it is probably the ignition switch. New ones are about $20. It could also be the wiring to the switch.
- If the same problem occurs, it is the solenoid. Datsun starters use welded connectors in the solenoid, so it's not easy to rebuild them. Just buy a new solenoid, or throw a good used starter on there.

If you don't have a remote start cable, you can use a screwdriver momentarily jammed between the starter "S" terminal and large terminal (battery +). Just be very careful, it you short the + to the metal of the engine or starter it can melt the screwdriver.

Of course there are other potential causes, some already mentioned. Best to test first, before buying new parts.