You can add product measuring units in system settings and then select the base unit while adding/editing the products. This feature will help you to manage the quantity well, while purchasing and selling in different units.
For example, you purchase coke as a box of 12 bottles and you sell them as bottle. You can add base unit of box/bottle and then add other with conversion. Add the products and select new base unit. You even can select the default purchase and sale units on add/edit product page.