Possession lets you take control of the player's next turn. If you buy Donate on the Possessed turn, your Possession effect ends before Donate triggers, so you don't get to choose what cards the Possessed player trashes.
So if I get this straight. If I play multiple Possessions and thus get to have multiple of the opponents turns in a row. The 'in between turns time' starts with the 'after this turn time', where Donate happens, and is still controlled by my opponent, because it falls outside of his (possesed) turns.
I find it confusing to know who is in control outside of the turns, because Donate just says 'you' are.
I think I got it figured out like this:
'You' refers to the one who plays and buys the card, which is in all cases the owner of the deck. It is just that (during the turn!) their choices are made by the player who played Possession.
Is this correct?
EDIT:
This weird case seems to fall within this explanation at least. B posseses Q. Possesed Q plays a lot of cards and buys some emporiums, B gains those emporiums without points, because B does not have actions in play.
Question: Would this mean that 'when you buy' effects happen to the possessed player, while 'when you gain' effects happen to the possessing player?