You can sell a single product (e.g. on Amazon) and this actually relates to multiples of either the same product or different products. The stock can be correctly managed and the packing lists show the correct details.
You can set this up in 1 of 2 ways:
1. If you amend the Product that you want to amend the stock details for and click on the Stock tab. You can change the pack size. This means that when somone orders 1 of the item (e.g. from Ebay) it deducts this value from the quantity in stock (e.g. 2).
2. On the Components tab on the Product you select the product SKU that the item sold is for and enter the quantity of that product. This is useful if you sell a product in singles, or multiples and when you sell it (e.g. on Ebay) it has a different SKU for each multiple. And when you ship the products you simply ship the same product just in different quantities.