It would just be a pure exploit on the part of the player. Mining bases flip very easily, so you could just make a secondary income at the expense of the AI by repeatedly selling him the same bases.
Planets at least have their own influence and can be made resistant to flipping. Plus you don't know what harm he might do during the period when he owns your planet.
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This Mining Base was located deep within their territory at the beginning of the game. The system only had two asteroid fields with no habitable planet near it, but one of my smaller planets and the AI home planet. The Mining Base was going to flip and I wanted to trade it when I ask them to be my ally because they were friendly already or like you said exploit some quick income to put in my bank. (Do not the governments in the real world exploit each other, when they think it will go unnoticed by the others?)