Telling them you have something when you don't can be even worse. You also need to interface with systems that let you know when to order new inventory, or order the raw materials you need to build more products for inventory. You also might want to interface with purchasing systems, so you can handle returns. These interfacing issues are generally out of the question with canned solutions. Even if they are possible, they require large amounts of custom programming.
Different businesses have different needs.
A car dealer wanting to sell on the Internet will have different needs from a grocery store or a travel agent. A company selling classical music CD's will probably want a different kind of catalog from one selling popular music. Even if a company's needs are exactly met by a canned solution, it will still want to customize. Of course not. And your needs will surely change over time as your business evolves and grows.