You could also have an un-balanced trade (I forgot what the technical term was). If Super Mario Bros. is selling for 300 points, and Zelda is selling for 600, the house (government) could cause the transaction to take place (assuming each wanted what the other was selling) and the person getting SMB for his Zelda would get SMB and 300 points to use in the next transaction.
The obvious answer is that the market place was already seeded somehow, for instance, buying points for half price.
The question is why would the market place do this? The obvious answer is to drive traffic, and not just for ad revenue. I don’t what the site looks like, but in today’s internet economy, traffic driving still isn’t enough to generate (useful) revenue. Though, it might be if the site owners have extremely low expectations.