It's a market. Japan in the series is very poor in the carbon market, so they use drastic measures like the supergrowth forests and military enforcement to keep the alloted carbon use low. That way, ATLAS and others can sell it.

So a country sells their quota to rich, polluting countries like the US or the EU. Those countries in turn are going back and forth attempting to offer the lowest price they can...that is higher than others. They need the quota, and they can't afford to spend to much, and the countries with a quota can afford to sell theirs for the highest price. That's where it gets way more complicated than a normal economic market.

Disclaimer: I'm quite sure that I am about to get something wrong in the following paragraphs. Do keep that in mind, I only learned this kind of thing from the radio.

It's not that different from oil commodities today. Companies like ATLAS (Ryoko) or Ishida Finance (Karin) will buy and sell these credits back and forth because they have enormous hedge funds behind them. They will sell them back and forth to each other or to other countries before they get to their intended delivery destination (so to speak anyway).

Ishida Finance and Japan bought a shitload of Kuwait, and since Karin is a market leader, everyone buys into Kuwait because it is rising so rapidly (like gold did a few months back). Only after everyone goes in and raises the price even further, Ishida Finance and Medusa pull out rapidly, crashing the price. They make off with a shitload, everyone else gets hosed. Serpent was doing the same sort of thing, but it was a lot more aggressive than Medusa or Metal-Age is, driving the country into ruin rather than just making a lot of money off it before allowing it to settle back down at a reasonable pace.

After that it goes above my head, but the banks falling off of Serpent like scales were like the recent economic crisis. They would invest their money into the fund that the Serpent was hiding as, and then as the Serpent starts going on a rampage to destroy the Kuwait market, most of them get taken out as well by the crashing of the market.