I think your test of plugging in the old solenoid pack outside the transmission was a good one. Since that solenoid pack only buzzes for a short time, it seems the replacement transmission may have a faulty solenoid pack.
Is the solenoid pack something you can replace without removing the transmission? It seems like that would be ideal, but if it's more work than you want to go through, I suppose you could disclose the diagnostics you've performed and offer the seemingly working old solenoid with the purchase of the car and let the buyer figure out if/how they want to install it.
-Rod