Dark Crystals Creating Negative Stat Items?

Nobody buys "100% overall dex" scrolls from NPCs one at a time if they are buying more than one. When you double-click on the scroll, a popup window comes out where you can specify how many of the scrolls you want to purchase. If he's buying 10 at once, you put in 10 and get 10 at once. So unless he had a 100% overall def in his inventory from a monster drop, no, he more than likely used 10 100% overall dex scrolls on the item.