I think there was a runescape update or something, the shop API doesn't work anymore.
It won't find the items I'm looking for anymore.
I'm guessing it's just a child ID issue, but I'm not sure.
EDIT:
..nevermind it's working fine now. Not a single idea what happened.
EDIT2:
Figured it out. Here's what happened: Created shop object.
Sold pure essence.
Since I created the shop object when there was no pure essence, it didn't register that there was now pure essence, so it was returning null.
I was trying to recreate the shop object, and that just messed all kinds of things up.
Could you add a "refresh shop" method maybe?
Or I'm wrong and it worked perfectly fine just...not for a bit there.