First out to try the new "buy" button in twitter are - surprise - musicians. You know, those stodgy luddites who can't adapt to the new world order of selling t-shirts instead of their song. Pitchfork reports that an initial group of artists including the New Pornographers, Pharrell, Death From Above 1979, Eminem, Ryan Adams, Wiz Khalifa, Paramore, Megadeth, and Soundgarden are using the button. Other quick to try are charitable organizations like DonorsChoose, Global Citizen, and (RED). What are they selling? T-shirts, do'h.
How does the buy button work? It's really rather simple, the app allows you to enter the shipping and payment information after you've tapped "buy" and once your order has been confirmed the info is sent to the merchant. Twitter's blog post about this test run doesn't mention if they're taking a cut, if it can be combined with Google wallet or what the future holds.... It does say that twitter keeps your shipping & CC-data encrypted in the app, which is also why the TOS changed today (you might want to read it).