Sales invoice - you select inventory-sales and then in the next column you select the inventory item that you want to sell to the client
Purchase invoice - you select inventory on hand (under assets), then you select inventory item from the drop down in the next column.
You should not be using stocks as the second column. That is what inventory-sales and inventory on hand is - stocks. What you should be using for the second column is you should be creating different inventory items like apples, oranges, bananas or laptop abc, computer xyz and Ipad MNO or whatever it is that you are selling. The purpose of inventory items is to have the prodcut such as an apple listed in inventory items with its purchase price, selling price, VAT inclusive or not as well as part code. Don’t use stocks as second column - it needs to be the actual product that you are selling.