Q. How do you sell Contraband directly to a base ?

A. You need to first send your Gold pod within 60 ly of the target base, then set the target base as the anvil object and select "Sell pod to anvil object" button in Pod Control. You might get lucky and this works. You should get the money to your HQ and the enemy base gets increase in crime if they have high desire for the items sold.

First, to trade hull plans, you load the plan on the a Gold Pod

Have the person your trading either transfer off the crew, or space them leaving none on the ship. No you can either transfer crew on or tow it.