hello i am a computer science student from Istanbul who wants to build an e-commerce website (not a big one).I am familiar with programming languages but i do not have much experience.there are some open-source portal systems that have modules for e-commerce or standby e-commerce systems like osCommerce written in php.Also there is dotnetnuke system build with .net but my problem is since i do not have much experience with these environments i think customizing/extending, writing modules for these systems seems extremely hard for me.Besides since these Content Management Systems are huge there may be a lot of bugs and many of the feautures i will not use..Is it better to build a system from scratch so you can have full control because it is yours and is this a very hard to thing to do or is it worth spending time learning architecture of these open source CMS projects and how to configure/extend it.Please give me some answers and guidance.Thank you.

I think starting from someone else's work rather than trying to build from scratch is the right approach. Even if you are looking at a bigger more complex system than you need, it's likely to be less buggy than something you build from scratch. And when you're talking e-commerce, you don't want bugs.
In addition to the well known portal systems, you can also look at some of the booksamples as starting points:
ASP.NET 2.0 Instant Results:
http://www.wrox.com/WileyCDA/WroxTit...echapters.html
has chapter 9, Wrox WebShop that is a ready to use ecommerce project. If you can't buy the whole book, that chapter is available as a PDF from us in the Chapters on Demand.
However, any modification to that is going to assume you understant ASP.NET at least at the level of
Beginning ASP.NET 2.0:
http://www.wrox.com/WileyCDA/WroxTit...764588508.html

There's a new DotNetNuke book coming that would be a great help if you go with that system:
DotNetNuke Websites Problem Design Solution
http://www.wrox.com/WileyCDA/WroxTit...470190647.html
The focus here in DotNetNuke as a business solution and building the whole site might be useful.

You mention PHP, if you're really more interested in that you might look at:
PHP and MySQL: Create - Modify - Reuse
http://www.wrox.com/WileyCDA/WroxTit...echapters.html
The chapter 9 project there is a shopping cart and you can also purchase a PDF of just that chapter.

Good luck!

Jim Minatel
Acquisitions Director
Wiley Technology Publishing
WROX Press
Blog: http://wroxblog.typepad.com/
Wrox online library: http://wrox.books24x7.com

thank you very much for your reply and information.