Most people posting in this thread probably already know this, but in case some new folks are reading along:
Inventory that is declared "belongs to" the collective
DVC owners. In principle, there should be a balance between the inventory "declared to DVC" and the points that have been or shortly will be sold by Disney. If points are not yet sold, but the inventory backing those points has already been declared, that's equivalent to Disney giving away something it owns and hasn't sold yet. In practice, there is usually a little more inventory declared than points sold, but only enough to support the sales of points for the next "little while"--on the order of months.
What's more, at the Poly, you are not buying "at the Tower". You are buying at the overall resort. Your points will be deeded to a particular, Unit, and that Unit will probably be in the Tower, but that's just for accounting purposes, it doesn't give you any special privileges in the long run for booking the tower.
Anyone considering buying Poly points should understand this--owning points at the resort gives preference only to the resort as a whole, not to any particular room type, the tower vs. the longhouses, etc. The only way to guarantee that you'll be staying at the tower (vs. the longhouses) is to buy a Favorite Week out of Tower inventory.