The best trade deal I have gotten recently involved a mining starbase I owned.
I had added enough modules to get it to +34, but had never put defenses on it -- I regard those as almost pointless at Suicidal as the AI sends big fleets. I was at war with two AIs and one slipped a fleet past my pickets just as I committed the local defense fleet to support the invasion of one of the AI's planets.
When the AI fleet got to the starbase, I traded it in a nice deal to another AI. Of course, I was careful to chose another AI that the enemy AI was also at war with ....
Then, when the AI fleet moved on, I re-constructed the mining base. Maybe I can do it again?
