Rubles Steam

Hello!

In General, such a topic.
I zaregalsya on Steam market and sold gear from Puba. With that money I bought another game (Bioshock) for 600 rubles.

I can't understand, who pays the manufacturer Bioshocka, because the money I got by selling virtual armor is not backed by real money.

Guys, anyone knows how this operate? How to pay for purchase of Steam-rubles? Why Steam allows you to buy their virtual currency the real game?

Thanks in advance!