Hi,
The one thing I am spotting immediately: The text line starting "Little Giant PondGlow underwater light kit...." is spanning the entire width of the page, and is apparently governing the width of the page. It goes to the exact margins on both left and right. That might be a clue to why the page is forced to that exaggerated width. There may be something in the code that's not allowing the text to do a normal wrap. I have seen a similar problem with use of no-break spacing.
This specific statement in the code might be the cause:
<pre class="standard1" style="MARGIN-BOTTOM: 0px; WORD-WRAP: break-word">
I generally know the Auctiva tags and IDs, and I don't recognize that as one (class standard1). I do see it referenced several times in the code. I've not used the HTML "PRE" extensively, but it suggests text within it's span is already preformatted, e.g. with returns. It's at least a candidate for the problem. All the document widths up to that point are set 95 - 100%, so anything that would push the document wide is suspect.
Danno
edit: "standard1" is set in eBay's section of the code (top of page in style sheet) to set the font size and family.
edit2: I checked one of your other listings, and that "standard1" class isn't used in the description (not an Auctiva piece of code). It appears to only be used in the Seller's payment instructions section of the listing, i.e. by eBay.