You said the snake is eating every week. It doesn't sound like it's being too finicky to me. It's likely stressed due to the enclosures and conditions that places like petco tend to provide. I'm going to assume from your original post that the BP is being housed together with other BPs that are marked to sell, and that they don't have much of a place to hide. Sounds like a stressed animal to me. Make the point to potential buyers that he will likely not be that way once set up properly and by himself.