Why are we basing this on the gox rate? I'm paying money for interest or whatever since his money was held up for a week, but the fact that we're using gox's rate (which is 10% above all other market rates) is a bit of a question to me.

The market has not gone up since he sent the coins, it has gone down.

I'd prefer if we picked any exchange other than gox to determine the amount of interest I owe.

Sorry for the belated reply, I'm not at home right now and as such am having a hell of a time keeping up with everything online.