still, this doesn't make sense to me. why does john not simply keep your money until delivery of the chips, and then pay the seller? why does it need to be the double amount?
the reason to use escrow is to wait with the payment to the seller until delivery, to keep up the incentive (payment) for the seller to deliver. why would he need to pay anything?
TL;DR: There's still a chance that the seller might sell the chips later on to another user if they offer a better price to him, and thus a deposit of sort is needed to disincentivize that.