When you receive an order that contains a product that has an individual serial number for that item (e.g. mobile phone), you can add that specific serial number to the order notes. This way you can keep track of what product was sold with what order.
Currently, Veeqo doesn't support serial numbers as part of editing / creating a product. If you'd like to request this feature you can add it by hitting the orange Suggest Feature button whilst you're logged into Veeqo.