I'm assuming if the "Add to Cart" button is gray beside a product, it means I already own or have preordered a product. It would be nice if that meaning was clearer, perhaps with a change in the button name or a hover message.
However, I noticed that the "Add to Cart" button is gray for the Pillar Acts volume, which I don't own. I did preorder it, but I canceled that order before it was released. Or perhaps it's because I own the Pillar set, but the Acts volume is not a part of the set.
