tradeEverything=false
Still, I wanted people to be able to trade the occasional base so I have added this...
Be an Arms Dealer
Who doesn't want to? You will now be able to sell ships (and bases) to other empires. Here's a screenshot of me selling a cruiser.

The price will be determined by several factors. The starting price is the CURRENT cost to build the ship based on resource prices. So build low, sell high. There are then other factors that go in to it. I'll leave it to the capitalist-minded among you to figure out what those are.
Procedure:
Select a ship or base you own and press CTRL-T (T for trade)
You can repeat as many as you like. Then, go to the diplomacy screen and open up the trade panel. All the ships you selected will be there. Once you leave the rade panel the list of ships will be cleared. So, before you queue up a large list of ships you might want to check that your potential trade partner has the cash.