Awesome project (hi Andrey!). Question: Why is a browser plugin required to access DApps like this? I don't immediately see a clear discussion of this on the web.
In order to interact with the DApp (buy or publish an ad), you can use a plugin or a wallet app like Mist in order to be able to send transactions to the smart contract. You potentially do it manually by interacting with the contract through a ethereum console and sending the right transaction to the right address, but the browser plugin / app makes the process seamless.
Metamask (the browser plugin) is one - albeit a bit clunky in terms of UX - way of reducing the friction of doing web based ethereum payments, separated from your main wallet