It is opt-out only and you have to do it in writing, there is no online hassle-free way to do it, which is on purpose. There is also no way to confirm they really stopped selling your data.
[1] http://www.spiegel.de/netzwelt/netzpolitik/datenschutz-vag-i...
Do you have a credit card, and have you read the pages of fine print?
To make myself clear, I'm not saying they don't do it. I'm saying it's probably illegal, therefore the fine print doesn't matter.
For example, you can purchase foodstuffs from vending machines using atom coins. The mind boggles.
Tip: Carry your atom-wallet in your front pocket. Pickpockets have a hard time with front pockets.