Hi Cleaver,
Thanks for posting and welcome to the Community Forum.
I understand how important it is that we record our inventory the right way. In my understanding, how an item is bought and sold can be set while creating your item. You check the buying and selling details tab of the item to make sure everything is set correctly. I recommend reading this article, Creating Items. This will give you a complete guide regarding how to set items.
Feel free to let us know if you require any further assistance.
Thanks,
Genreve