Wrong, when you post an order-proposal in PurseIO or Broker, you're putting a sale proposal, not a sell order, I mean, it's not a sell order in an exchange. When the buyer orders your items, it's basically an immediate-buy-order. So the last step is the one that matters, the person that confirms the transaction is in this case the buyer, as opposed with BitPay-based merchants (that convert immediately to fiat).
My bad!
I wasn't aware how actually Brawker worked. I thought someone will buy on your behalf in exchange for a small fee (if that was the case, he will need to sell BTC for fiat to be able to make another purchases).
But apparently it works the other way, it matches people who want to buy BTC with those who want to spend BTC.
Here's a short video explaining the idea:
https://www.youtube.com/watch?v=M0-aZit2oI8