If you already have an offline business you may think that you are missing the boat by not selling your products and services online. This could be true if your products are a good fit for online marketing, but some products just don't sell good online for one reason or another.
One of the reasons that you may have a problem selling your products online is that marketing online is a little bit different than selling from a physcal store front. When you are selling from your store you can present your product to your customers in person. Online sales requires a little bit more build up to gain a customer's trust.