Testing with Odoo 8.0-20150411, it appears that I can add any product to a sales order, regardless of whether the product record has the "Can be Sold" flag set.  I note that when adding line items to a purchase order, I can only choose from items that have "Can be Purchased" set, which makes sense.

If would not want users to be able to accidentally sell items that are mapped to the wrong G/L accounts because they've inadvertently selected an item that was not intended to be sold.