Types

Basically two types are found based on the ownership and control of the portals.

Under the first category falls the group of websites which are owned and controlled by the Businesses who display and sell their merchandise and products directly to their customers and other people browsing the Internet. Here, the entire system of inventory, sales and all other related aspects of the business are handled by the business owners themselves. So huge investment and proper supervision and control and risks associated with such transactions have to be tackled and managed by the owners.ExampleTripleclicks

Second category consists of those websites where the Third party sites are used to sell  the products of a business firm. But the cost of investment and risk of administration of the web portal  is reduced but this comes at a cost which is paid in the form of a fee/commission to the website owners. The business firms have to simply plug in and sell their products.Example:Amazon.com, flipkart,  Tripleclicks

Another classification can be done as 

1. Auction sites: where branded , new or old , items are offered for auction.
2. Rental sites: here properties like Travel gear, vehicle, etc... Are available for rent 
3. Sale sites: products or services are sold here 
4. Referenced sites: these lists out and recommend a host of sites from which you have to make a choice.
5. Niche sites: here the specialist products are offered.
6. Ad sites: where you can place your ads to sell your products.

The Third group is like this.

A set of Online shops only  are available and no offline and physical shop will be there.

But some business manages both the online and offline shops.

Many mega mall stores have opened their online shops to cater to their customers and the net browsing people while some sites run their shop only online and no offline shops are allowed.