You will want a web-store that can support your business as it grows. This means you will need an application which will not “slow down” as greater demands are placed on it; and it also means that you will need to keep in mind growing data transfer needs as your requirements grow.
If you anticipate rapid growth of your store and your traffic levels, you would probably be well advised to avoid some of the cheaper web-store applications. For instance, there are many cgi-scripts available that will provide shopping cart functionality. Many of these are free, and may even be supplied by your ISP as additional ‘freebies’ to tempt you. While these are cheap, there are limits on their speed, and they work well only with lighter levels of traffic.