Update: It was fun while it lasted, but just like that, the purchase links have been made invalid. The SteamDB listings are still in place, however.
Update 2: At least some of the Destiny Silver purchases made through Steam (including mine) have been automatically refunded. Still no word from Activision about why it was listed in the first place, however.
The good people of NeoGAF noticed something strange and interesting earlier today: Silver, the in-game currency used in Destiny's microtransaction system, can now be purchased on Steam. It's listed as an "unknown app" on SteamDB, meaning that it's "been deleted or is hidden to the public," but hitting these links—one Silver for $0.01, or in amounts of 500, 1000, 2000, or 5000—will take you to a functional purchase page, where you can spend some money and end up with a new line in your purchase history to show for it.
I tried it and it works, although as far as I can tell there's nothing in my inventory reflecting the purchase. But my big two-cent buy did result in an entry in my purchase history for "Uninitialized," underneath which it states "Destiny Silver."
The SteamDB description describes the items, which include bonuses for purchases of 1000 or more, as "an in-game currency you can use in Destiny to buy goods for your character in the Eververse Store. Visit Tess Everis in the Tower to see her available inventory. Use of Silver is subject to Bungie Software License Agreement."
Of course, Destiny is not actually available on the PC, and while Destiny 2 will be, Activision said in May that it's coming via the Blizzard launcher (Battle.net, as it used to be known), rather than the more ubiquitous Steam.
So what does all of this mean? Quite likely nothing; lots of things happen in the background on Steam, out of sight and mind, and this could well be just a bored Valve tech doing some testing of something entirely unrelated. And as exciting as the prospect of diving into the original Destiny on PC may be, that doesn't strike me as especially likely, if only because I would expect Activision and Valve to do a better job of revealing it to the world than tripping over a leak on GAF.
On the other hand, there it is, "uninitialized" perhaps, but active enough to take your money. I've reached out to Activision for more information, and will update if and when I receive a reply.