Ok, here's my submission. It's a cross between smugglers and duplicate.
I think this amplies the problematic scenarios of Smugglers where it can cause a stalemate (ideal for both players not to gain anything). Unlike with Smugglers, you'll always know exactly how much benefit your opponent gets from you gaining a card. I'd imagine that in a lot of situations where this is the only way to gain cards, the game breaks down.
I also disagree that this is worse than Duplicate without the +1 Action; Duplicate only allows you to gain two cards of a kind at a time, whereas this is more flexible, and you can get the benefit even when you wouldn't have been able to afford the thing you want to gain.
Thanks for the critique. The more I think about it, the more I'm certain you're right. I'm imagining where 2 players each have 5 eavesdroppers on their mats and have reliable engines that could play all their eavesdroppers every turn. Every card I buy is 5 copies for my opponent: Stalemate.
I was also bothered by another problem. This might work ok in a 2 player game but it gives an advantage to the next player in a 3 or 4 player game. Say player B and C each have 5 eavesdroppers on their mat and there's 6 grand markets left. Player A buys a grand market -> Player B gains 5 grand markets (C doesn't get any).
So I've made a small change that allows only one copy per card gained (that way it's 1 to 1). I feel this solves both problems. In addition, I felt I had to decrease the cost because of this and I thought I'd take off the +1 action to balance it (it was bothering me anyway. it felt like it wasn't in the duplicate and smugglers family if it's non terminal.)
Now it cost the same as smugglers but they each have their strength and weaknesses. Eavesdropper is more flexible, but you can't use multiple on the same card, and unlike smugglers you can't use the card you gained on the turn you play it. In addition, because eavesdropper is a reserve, it (and the card it gains) are more likely to miss the reshuffle.