So I guess Messenger/Gardens will be a thing?
In a Gardens mirror, the on-buy is a wash because everybody gains the thing. It may be better if the others aren't going for Gardens. Messenger could help with emptying piles, and it comes with +1 Buy, so there's that too.
With Watchtower in hand, this is a cheaper Ill-Gotten Gains.
Not quite... you end up with one Curse in the trash instead of still in the pile, and Messenger in your deck functions pretty differently from IGG in your deck.
I'm tempted to think it has the Ironworks-type ruling.
1) Buy Messenger
2) Attempt to gain Curse
3) Reveal Trader, gaining a Silver
4) Continue with Messenger: "Each other player gains a copy of it". There is no "it", because Messenger did not gain a card.
I'm like 0 for 100 on these ruling questions, though, so a good rule of thumb would be to assume the official rule is whatever isn't what I'm saying.
I agree with this. Trader makes it so that Messenger failed to gain a card itself, so there is no "it" to copy. That's how it works with Ironworks as well. The salient points are the "would gain" on Trader and the "it" on Messenger/Ironworks. I don't think the "if" on Ironworks factors into it.
Also, this is a really neat card.