Pollution Allowance Auctions

In high school debate, twenty years ago, I ran a case for auctioning pollution permits, the application of the free market to pollution. We did pretty well because there was nothing written against it. In the last week, it's hit the headlines. Wired points out that sulfur dioxide went on the market in 1993. Paul Krugman argues that the market fails in the case of local pollutants like mercury (though his research has been questioned). And after reading WorldChanging's take on pollution permits, I have to wonder, why aren't these sold on E*TRADE? If I want to take 5 tons of pollution off the market, why should I have to go through a broker? And if I buy 5 tons, what stops Congress from releasing 10 more tons tomorrow?