It certainly seemed to be feeding on So-Yeon, but I'm not sure if that was because of the particular bargain she struck with it, the Orb's nature, or just because she was keeping it within herself.

Perhaps, but- So-Yeon stole the orb, and was able to make a bargain with it anyway. But that might depend on what the orb sees as meeting a bargain.

[And some morbid part of Sabriel wonders- would the death of the current owner terminate said ownership?]