Ah, thanks for the answer. That's a bit disappointing, but I understand that it probably makes the game much more balanced.
I still think the "need to put something up for the taking" is a bit inconvenient in multiplayer. There's a way around it (striking a deal with another player via chat and only then completing the trade in-game), but a bit inconvenient that you can't designate the player who is allowed to take the Great Work. I imagine it would be annoying in games against strangers, when you put up a GW with a certain player in mind, and another grabs it before the intended recipient does. I mostly play against friends, so it's not a big deal, but I think it would be nice if this was addressed.