If you move equipment from a pet's slots into the selling section of the inventory and then click sell it throws an error that there apparently isn't anything there to be sold. You have to move the equipment to the main section of the inventory first as an intermediate step and then from there to the selling section for it to work.

This is working as intended with how the system works. Until you put the item back into an Equipment slot, it is still considered equipped by the pet. An item only becomes unequipped when it is out of a pet's 'inventory' and into an equipment slot. And only at that point, are you allowed to sell an item.

Thank you though for the heads up.

Ah, I see. I guess this is a suggestion then, as opposed to a bug report, but it would be great if you could take items off of pets and move them directly into the sell zone. I was selling a whole bunch of equipment off of pets yesterday so I could sell the pets, and it gets quite irritating having to move them all to an intermediate point first when you're doing a lot of them.