The most important thing about eComm sites - way above "s e x y" looking design - is USER EFFICIENCY.

Webshoppers want SPEED, SIMPLICITY, EASE and EFFICIENCY. (These FOUR key factors were identified in a massive research undertaking conducted by the eConsultancy a couple of years ago.)

So... anything that makes sites SLOWER, MORE DIFFICULT, LESS STREAMLINED and LESS EFFICIENT, need to be addressed.

By making your site 100% width, you are "stretching" various components over a wider area - and one result of this is that users need "more mouse effort" to navigate the site.

Think of it this way...

If your competitor's site achieves a user goal in TWO CLICKS and your site achieves the same goal in THREE CLICKS, your site is 33% LESS EFFICIENT than your competitor's site.

This stuff is known to have a huge impact on sales. Less efficient sites will not get as many sales as more efficient sites...

So... while YOU make like 100% width, please know that it makes the site LESS EFFICIENT and MORE DIFFICULT to navigate.

But don't take my word for it... when you aren't getting sales, it's probably because your visitors (potential customers) feel the same way.