A mysterious InTrade forum user (could be a trader or could be John Delaney) has posted this:

*****
Newbie

Joined: 24/01/2010 15:58:35

Messages: 1

Original thread at midasoracle.org:

So the actor has to a) not care about the transaction fee and b) have limitless margin. Intrade fits the bill for both of these. a) they don’t care about transaction fees because they are ultimately collecting them and b) if you only short when the bids are summed to over 100, it’s essentially an arb.

Until mid-April 2010, Intrade will refund market taking and expiry fees for arb trades &#8211- details here.

Some products with a single guaranteed outcome are linked for cross-margining purposes. If you collect at least $10 by shorting all three contracts for the 2012 presidential election, then you will not have any funds frozen. The contract rules will tell you if a product is not linked (e.g. 2012 republican presidential nominees).

Intrade provides an API for developing trading applications. I am running a bot to take out market imbalances and as far as I&#8217-m aware Intrade is not competing with me.

Ah.