From what I can tell, creating an inventory item that has a suitable description for both sales and purchases is not achievable.
If there was a tick box under the item details tab for:
'i...
This is something that most other packages (some that are also much cheaper) are able to do. ZOHO does a great job of this where you can even include an image of the item along with being able to make a copy of an item to create another (such as a different colour or size).