He also will not buy: books or paper (other than scrolls), non-armor apparel (even magic rings and amulets), light sources, or alchemy, armorer, or thief tools.
As a creature rather than a humanoid NPC, Creeper is one of the few merchants who will purchase skooma (but not moon sugar, since he doesn't deal in ingredients).
Creeper cannot be pickpocketed attempts to do so will merely open his dialogue screen.
See the Hints page for ways to make up for this.
His 5000 gold is not enough to get the full value for some of the more expensive items in a single sale.