You'll gain stacks of Spellbinder Orb after you undo a purchase of it.

Here's how I created the bug. Bought Spellbinder Orb while dead, then undid the purchase. When you're alive you'll find yourself gaining stacks of the buff it provides.

The buff itself does nothing and only works for the items' activation.

Did not test to see if there were other ways to gain the buff or if you could activate the item with stacks gained this way.