This is smart but I'm not sure what the fundamental transaction on such a market is. Keep in mind that there is no rivalry for karma (in an economic sense) and that people always keep their karma after giving to others (or possibly even increase it).
Not sure I agree; I think people are already using ad hoc methods to "price" karma; for instance, I have used such methods to "price" a Hacker News karma point as 1,000,000 reddit points.
The side effect of the market would be to rank discussion systems by credibility, or signal/noise, in real time.
It's also geeky enough that I think you could keep liquidity, even though there's no dollar value to karma.
