About the planetary scale, does it really matter ?
if we were still a feudal society and if intergalactic, it wouldn't be far fetched to sell & buy planets as a whole (perhaps not EARTH and the like though)
imho, this problem about AI selling himself off is of 2 scales :
- 1 the AI that is 10x inferior to you : no issue here, if he's that far below you could probably win all other kinds of victories
- 2 the AI that is at your level, barely inferior or superior : here it's an issue
imho what would need to be done is :
A - weight down monetary propositions according to AI economy & treasury & needs : that is if the AI earns 1000bc a turn, then your offer of 2000bc is just 2 turns of income, not that incredible anymore (if the AI already has 20000bc surplus in treasury then it should again give less value to your offer) (if the AI is at war and will probably need to buy dozens of warships then your money is more welcome(military rating comparison)). EDIT: Also selling a planet which is just beginning could make sense if the AI is struggling economically or if the planet is in danger of being invaded/turned, otherwise not so much.
B - weight down any property propositions according to AI state & number of objects in galaxy : same as above but for planets, giving a planet when AI has only 1, is awesome for him, but doesn't increase his chances when another player has 100 planets under his control...
I know it probably can't be done through modding and that's unfortunate.
still such drastic options should be left optional in my humble opinion. that said I'm not even using that patch so who cares about my opinion