Bob gains his farmland back form the trash.
...except that Bob doesn't gain the Farmland. It just teleports to his discard pile magically.
To be even more nitpicky, Bob's farmland is never trashed. It is instead set aside, then discarded at the end of the Possessed turn.
This is not true. Bob's Farmland is trashed (and Bob could discard a Market Square for Alice to gain a Gold), and then it's set aside, and then it's not discarded, but rather it magically teleports itself to the discard pile (which arguably matters if someone trashes a Tunnel during a Possessed turn).
Whoah. That leads to another set of weird edge cases:
Graverobber and Rogue. If Alice possesses Bob and plays an upgrade, trashing his duchy to gain a gold. Can Alice then play Bob's Rogue to gain the Duchy from the trash (causing possession to "lose track." Or is a trashed card still "trashed" but then immediately in this mystical "set-aside" stack? (never in the trash pile)
Assuming that last mechanic is correct, a trashed card is "set aside" for the remainder of the turn. What happens if a possessed player trashes a fortress? Does it go to back to hand (text on the fortress card) or into this "set aside" pile (text on possession).
And what about Forager? If the trash is empty, and I play forager on a possessed turn to trash a copper, is the forager worth 1$, or 0$?
I'm pretty sure about this:
Any card that is trashed visits the trash pile, but instantly leaves it and gets set aside. Just like when you top-deck a bought card with Watchtower, it does visit the discard pile for a moment.
So no, if you have someone trash a card while possessing them, you cannot gain it by having them play Graverobber or Rogue. And Forager will not see that particular treasure card in the trash pile, because it will be gone before Forager is played.
As for Fortress... you trash Fortress while possessed. At that moment 2 different triggers go off, Fortress's and Possession's. You would choose the order that they happen in, but because you're possessed your opponent chooses the order instead. If they choose Fortress first, then the Fortress ends up back in your hand, and then when Possession's trigger happens, it has lost track and cannot find it to set it aside. If you choose Possession's trigger first, then it gets set aside, and then when Fortress's trigger happens, it last lost track, and so does not return to your hand.
*Edit* Ninja'd by all the same answers.