Onderzeeboot
Ecstatic Orb
I think you're really close to a cool mechanic, but I don't like that the relevancy of the market is dictated by the colors you and your opponent are playing. I would make cards in the marketplace effectively cost colorless mana, like Hostage Taker, and then balance around that. So...
Wage Theft
Instant
Counter target spell. If that spell is countered this way, put that card onto the market instead of into that player's graveyard. (Any player may cast cards on the the market, and they may spend mana as though it were mana of any type to cast those spells.)
Wage Theft
Instant
Counter target spell. If that spell is countered this way, put that card onto the market instead of into that player's graveyard. (Any player may cast cards on the the market, and they may spend mana as though it were mana of any type to cast those spells.)