When you're listening to a track, there is a link in the player bar to buy it. It looks like this: 
To take this feature out for a spin, go to one of the User Sites, open the player all the way, and hit the Buy
button. A new window will open with an Amazon search for the track metadata.
The query
The search uses whatever text the publisher provided to explain the track.
If this is a song, the text is often in the element content in the form artist - song title
. For any commercial release an Amazon search on that text is likely to provide results for a CD and for an MP3 download, which is pretty much ideal. For example, here are the results of a search for "Radiohead - Creep"
This method of matching tracks to products is flexible enough to cover all sorts of media. For example, the search results for "Allen Ginsberg - Howl", which is a Beat-generation poem, include a CD with a reading of the poem, an MP3 download of a reading, a paperback book, and a nonfiction book about the Beats.
Warning: a product search isn't always going to work. With net-native media like podcasts, for example, there usually isn't a product directly associated with the title of episode. And even with things like musical recordings that almost always have some kind of product associated, the search can take a bit of tweaking. "Club des Belugas - It's A Beautiful Day" returns no results, but "Club des Belugas" does, and the results include a CD with that specific song.
Affiliate program
Given that there is money changing hands publishers should be able to get a piece, so we have implemented an affiliate program: a publisher who is part of the Amazon affiliate program can earn referral fees on business they create.
What you do is sign up for the Amazon program, get your affiliate code, and provide it to us in the markup for your page. We'll pick up your code and add it to the Amazon link, so that any purchases from a clickthrough in the player on your page are credited to your account.
The markup uses an HTML meta element:
<meta name="amazonid" content="...your Amazon ID here...">
Note that this has to be in the HEAD section of your HTML. That's a bit harder to modify than other parts of the document, especially if your page is in a hosted environment which doesn't allow you edit the HEAD. This is an intentional security feature. We didn't want to create an incentive for spammers to insert their affiliate codes in other people's pages.
Philosophy
What's interesting about this button is that it automates a crucial but neglected part of the ecosystem for net media. Monetization shouldn't be a manual operation.
Music bloggers, for example, usually have a little blurb in their sidebar encouraging visitors to buy recordings associated with the songs they post. They do this out of a kind of musical environmentalism concerned with ensuring a flow of recordings to discover, keep, love, and post.
However, it's a lot of labor for a blogger to set up a purchase link for every track. An upsell link needs to be part of their infrastructure along with comments, RSS, and other basics. With this feature we're making purchase flows a standard part of the tools for web media.