From my understanding, it is a private exchange between the developer and the mitigation bank. More or less, the mitigation bank's analysts determine what they think the credits can be sold for, and the developer decides whether or not the credits are worth the asking price, based on how much the regulator told them they needed to offset.
The more competition for the credits, the higher the price (obviously.) A lot of the business is speculation because the developer of the mitigation bank has to determine a suitable location for the bank (in theory in the path of "progress") early enough to purchase the land as cheaply as possible. They then have to start the process of establishing/restoring the wetland and hopefully complete the process (which involves the US Army Corp of Engineers, US Fish and Wildlife, etc) before developers of housing tracts/shopping centers/etc have to start offsetting their impact. The more ecological impact, the more competition there are for credits.
The buzz I heard back in '07/'08 was that West Virginia corridor close to DC would become a great market for MBs because it was assumed that many beltway people would be trying to escape the Metro Area via West Virginia (instead of MD and VA suburbs which were already congested.) The recession obviously hampered development, though.
I actually went to visit a potential banking site back in '08 with my fiance when we were on our way to the Outer Banks in North Carolina. It was pretty cool to see the potential (it was just old farmland with irrigation ditches everywhere.) Unfortunately that deal died with a few others when their primary backer MuniMae (for which my fiance still works) decided not to proceed with the projects due to economic issues (both externally and internally.) The guy who was the primary brains behind the operation ended up buying the business entity and decided to pursue it on his own. They were called MuniMae Sustainable Land Investments (
Sustainable Land Investments - which is now defunct.)
I called my fiance at work and was grilling her with questions but she told me to leave her alone, lol, and that she would talk with me about it tonight when she wasn't in the middle of working. I'll hopefully be able to get a few examples of functioning banks and direct you to them. They're definitely in existence, and the model supposedly has worked many times before.
I'll keep you posted!