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