If you gave us some more information, that would be great and it would make it easier for us to answer your question.
You say it's a webshop, but you also mention Amazon and forums? And what language are you writing this in?
If you're doing it with Joomla, it's not from scratch. But I wouldn't suggest writing a webshop from scratch unless you've got some specific requirements anyway. If you're using PHP, I would either look at Magento or a solution with Drupal (they have a module called Ubercart that may be interesting.)
Magento (which I use for my own shop) is very powerful though and requires a bit of resources from your server, and it can be difficult for a newbie to "get into". On the other hand, it's super-packed with excellent features by default, and has a great community. Drupal, like Joomla, is a CMS that can be adapted to become a webshop as well. Why Drupal instead of Joomla? Well, it's just my personal preference to be honest.
If you're doing this in Ruby (which I've converted to using since the last 6 months), there's a nice platform called Spree Commerce that is very lightweight and easy to build your own stuff on top of. And of course, you can always roll your own solution with a framework like Rails or Sinatra to get everything exactly like you want it. My latest site I built with Sinatra, and it's fantastic.
But it really depends on the specifications regarding your projects, so tell us as much as you can and we'll be able to answer you more accurately.