YouTube has decided to make the Merch Store feature available to all Partners, instead of just Musician channels. Just don't be in a big hurry to set up shop...
As with various other Partner features, the Merch Store availability is being rolled out with time. Availability to the newer group of Partners (Partners made after the
video monetization change rule) is currently unknown (though I've seen
some claim to actually have it, I just haven't researched into it
I had expected that this meant most Partners would be waiting as long as the wait for YouTube Live, but that doesn't seem to exactly be the case. My account has access to the feature, and I've taken a quick glance at it. You basically choose to search for items available through the store affiliates, and then add them to your store. Each search is limited to the maximum of the first 100 results, though this shouldn't be a problem if the description of your product and your search are specific enough. (A bit of a funny thing, the default search query is 'None'. This actually searches for results that match the actual word, "none".)
You could technically sell anyone's merchandise from the partnering affiliates in your store, but since the revenue is handled through the affiliate, and not YouTube, you wouldn't be benefiting much from promoting a product that isn't yours.
Another point to note is that CafePress has been added to the list of affiliates. This adds them to the affiliate list, already including Topspin, SongKick, iTunes, and Amazon (for digital audio and video downloads, CDs and DVDs).
You can read the full Creator's Blog entry here.