E-commerce is the biggest trend right now in marketing products and services and whether you choose to get your products directly from a producer, or go through a distributor, or you may even produce the products yourself; you should consider the following:

Suppliers - without good suppliers you cannot offer products.

Pricing - a big part of e-commerce is that price comparisons are extremely easy for the consumer. Compare your prices to what others are offering.

Marketing - You need to figure out a way to get people to come to your site. If no one knows that your site exists, you will never sell anything. Try submitting to search engines, issuing a press release, combining your services with that of another or tying in with a charity.

Site Design - As in any site, make sure you offer a variety of ways to reassure your customer that you are a secure site and that you are trustworthy. Offering E-mail, FAQs, knowledge bases, forums and chat rooms helps you differentiate yourself from the competition and tells your clientele that you are open and would like them to find out more about you.

Customer Service - fulfillment, returns, customer service - these dedications make or break any retail establishment. They define your relationship with your customer. Stats show that 95% of the people who come to your Web page won't buy anything the first time. They have to become comfortable with your site, used to how you do your business, and feel secure enough to commit their hard earned dollars. If you have reliable and useful content, they will return, with credit card in hand.

Orders - You need a way to accept orders and to accept the money. Online payment processing has some consumers a little nervous; make sure you offer a secure process. Reassure your consumers that you are legit and trustworthy. Payment is usually done by credit card and issued before delivery.

Delivery - Instead of big on-line retailers selling direct to customers, which is how it was generally assumed E-commerce would work, distributors or drop-shippers are playing a big part in assisting online commerce, while the sales side is being handled by thousands of small affiliate businesses.

Returns - You may or may not charge certain fees for returns, and you may or may not require the customer to get authorization before returning anything. But you should always accept returns to keep a good relationship with your customers. Honor your warranties, even if you assume the cost of the return to the producer. Be clear on your site as to what your conditions are.

Customer Service/Technical Support - On-going products like cell phone service may also require on-going customer service because customers want to change the service they receive over time. Traditional items generally require less support than modern electronic items but both are equally important. Most companies offer a 24 hour support service.

****

Starting an Affiliate program is a simple way for you to get some extra cash coming from your Web site. An affiliate program is where you partner with another company, usually an e-commerce company, to sell their product from your web site. For every product you sell, you get a portion of the proceeds.

Some of the bigger companies (ie: Amazon)  encourage their customers to review their products and provide live testimonials. Some customers will return to the site, simply because of the reviews and other resources. The main issue is to make your customers feel comfortable and confident that they can trust you, give them a reason to return.

Another interesting feature that sites are now offering is the ability to customize the site to your personal interests. You can do things like create a "favorite artists" list so that whenever they have new music, you will find out right away. They can optimize their searches for products by categorizing them and returning at a later date to review them. 

According to emarketer.com the biggest product categories that sell online are: 

Computer products
(hardware, software, accessories) 
Books
Music
Financial Services
Entertainment
Home Electronics
Apparel
Gifts and flowers
Travel services
Toys
Tickets
Information