How does overpay work now? Based on what Ingix wrote, it seems the actual overpaying happens when you pay, before when-buy? But I don't think that makes any difference, I mean whether it's on when-pay or when-buy? Well, it could make a difference if you play with old versions of cards like Haggler; on when-buy you could gain Silk Merchant and have more $ to overpay with.
The ability that you overpaid for now happens on when-gain. So I guess this means that when you overpay, you choose how much, then you gain the card you bought, then the overpay ability happens? With Stonemason, you choose how much to overpay, then gain Stonemason, then choose the cards to gain?
How does this work with Possession? So I guess you set up a future when-gain ability when you overpay, for when you gain the Stonemason that you paid for, for instance. Something like, "the next time you gain this, gain two action cards each costing the amount you overpaid." That seems to mean that neither player, Possessor or Possessed, gets the overpay ability, since the player who overpaid never gained the card?