Bad Starter or Starter Solenoid?

When I try to start the car, I can hear the relay click/thunk, but nothing else happens. I've had my battery tested and charged and its supposedly good.

I jumped the two posts on the starter relay while the key was in start; I got sparks, but the starter didn't fire.

I grounded the NSS terminal and it did the same thing. I jumped the terminals on the starter itself with the key in the run position and got sparks, but no fire. It is a new mini starter that I bought for my Doug's headers.

Could it still be my wiring or is it most likely a bad starter solenoid? One thing I did notice was the the smaller of the two posts on the starter will tighten to a specific spot and then will just spin. The wire is secure, but I wonder if there's something wrong on the inside.