This is an interesting idea, but I just want to say something.
I am NOT a slave.
Element players may not get their final say in what element they're picked for technically in a war, but using the word slave implies that we're being forced into this war by crude means, and that we're generally 'lesser' beings, which I think is completely wrong. It's like I'm showcasing myself just to get selected. I mean, since it's a war, doesn't this make this a 'drafting' process? So a "Mercenaries for Hire", rather than a Slave Market, sounds better.
On another note though, the concept of 'bidding cards for better players' is intriguing, but there should be 1-2 members who get a 'bypass' for an element (usually those with major restrictions as said in the Suggestions and Feedback, such as being a forum newbie or not having participated in any PVP events yet).