If you want to use sharepoint (I have just delivered a job site, and a media advertising site using it) this is what is possible ...
It is a real pain to change the look and feel(master pages), and you have to get used to writing web parts for stuff. The learning curve is steep to begin with for example, instead of developing normally using Visual Studio on your desktop and deploying via source control and continuous integration, you have to download a MOSS 2007 image and develop your site using a combination of Virtual PC, Window Server 2003(VHD), Microsoft SQL 2005(installed on VHD) + Microsoft Office Sharepoint Services 2007 etc. This chews up resources like you wont believe. Also some of the development happens in Sharepoint designer, which is a yukky successor to the old MSO Front Page. You can use Visual Studio for everything but then you would be missing the point of going with Sharepoint in the first place.
Here is where you can download the image to have a look around ... needs WIN2003 activation though
Here is where you can have a look at a video which shows you how to develop ****point using Visual Studio
I wish you the best of luck & honestly if these are e-commerce sites to begin with I would definately consider going down the custom route. Your biggest enemy is the time it takes to learn the object model of any CMS vs. being short of time and having to code around it.