As everywhere else in Africa, ecommerce is booming by leaps and bounds in Tanzania and a few websites have been able to establish their leadership in the market, either by being the first movers, or having strong (overseas) backing that allowed them to spend on marketing both online and offline. Very few Tanzanians still buy or shop online, however the following companies/websites are the ones that will be in position to get the most of the market once it becomes huge.
#1: Kaymu (www.kaymu.co.tz)
Self-described as the #1 shopping community in Tanzania, it is the Tanzanian extension of the ecommerce platform launched by the African Internet Group in December 2012. Through Kaymu, individuals and businesses can sell their products online in a few steps.
#2: Kivuko (www.kivuko.com)
The Tanzanian online shopping gateway. It is one of the first ecommerce sites in Tanzania through which a wide variety of products are sold. Like Kaymu, it relies on vendors who can sign up through the site and advertise/sell their products through this platform. Multiple options are available for vendors at different pricing and functionalities.
#3: Jumia (www.jumia.co.tz)
The #1 online shopping mall of Tanzania. At the contrary of the first two, Jumia does not allow vendor registration but rather sell the products themselves (I guess they have a purchasing department that deal with vendors). Jumia originally was a Nigerian ecommerce startup which is now present in the UK and 10 African countries including Tanzania.
#4: Inauzwa (www.inauzwa.com)
A fast growing ecommerce platform in Tanzania where anyone can register and sell stuff in a matter of minutes.
#5: Shopping Tz (www.shopping.co.tz)
An online shopping website offering products from the UK primarily (or so they claim), mostly clothes, watches, flagrances etc.
Hope you will find this list useful. Ecommerce is a quickly evolving animal so this list is accurate as per the date of this post (September 2015), and may not be valid a couple of months from now.