We just launched v2 of our website and I've gone through 8 months of all kinds of questions like this myself. The best way to do it is to separate your tables into products, inventory and listings.
Products holds product specific information only, things that will never change. Inventory holds information about current inventory only, ie quantity on hand, backorder, location in warehouse etc. Listings are how you handle the multi item support. This table would hold listing information and draw from the other two tables for the product and quantity data. So you'd have a basic table structure something like this
Products
- productId
- productName
- productModel
- productDescription
Inventory
- inventoryId
- productId
- quantityOnHand
- location
Listings
- listingId
- inventoryId
- quantityInListing
- numberOfListings
- price
Our company sells a lot of preowned things so the listings change regularly, but you could always set the listings up to auto-populate based on inventory if you're selling new things that you just restock. Doing it this way means you don't end up with a ton of data duplication as you would if you just created products for the multiples. It also allows you to sell any item in any quantity and set prices for those quantities manually.