Using sales channels as price tables breaks add to cart

[jonmast]

Describe the bug
Using the functionality I added in #32 breaks adding items to cart. I discussed it with Luiz Xavier and he said only plain price tables can be used in this way, it won't work with sales channels. I'm not sure if there is a way this can be made to work but if not the feature should be removed from the admin.

To Reproduce
Steps to reproduce the behavior:

1. Add a sales channel (trade policy) as a price table on a B2B organization
2. Log in under that account on the frontend and attempt to add an item to the cart.
3. Add to cart request will fail