I want to echo Roland,
There are various e-commerce platforms that are "OPEN SOURCE" out there, Open Source simply means that they are under the General Public License (GPL) that are free for you to use.
For example there`s OSCOMMERCE 3.0, which is a very reliable shopping cart system that has been around for years now, then you have ZenCart, which is clone of OSCommerce with a few extra added modules that can be customized depending to your needs and that it is very stable and reliable, not to mention easy to customize "Of Course you need some experience"
Then you have a new shopping cart system that is taking advantage of PHP5 and MYSQL5, it is called Magento Commerce, which is very stable!
All you need is a hosting company that can handle all the new scripts..
As for your Inventory control system? Well I would suggest keeping an external database management system for that. You can still Categorize your items on the shopping cart for example you would assign a SKU number to your product, and when they place an order you punch the SKU into your Database Management System, and it tells you which shelf it`s in..
Remember, a shopping cart is a shopping cart is not a DMS.. but you can still keep track of your inventory.
Another thing, I wouldn`t advise for you to jump on the bandwagon of trying to design your web site, you should just pay for that service and have someone develop it for you "Professionally".. a little investment can save you thousands down the road.
Web Developer / Owner / Consultant
When starting your own business the need to "know-how" is greater than money!