Many companies are using the internet now to advertise and sell their products, making it much easier and faster to do business. There are a lot of platforms to use when dealing with eCommerce. If you are looking for a small cart on your website, Google Checkout may be a great option for you, but if you are looking for a large scale eCommerce system you would want to go with something like Magento or osCommerce. Both have been around a while and have great track records. Not only are they the best, they are free to use(unless you buy the corporate licenses).