Iframe has nothing to do with it. If you take a look at the page source you see the ebay stuff consists of a simple css tag containing links to some ebay javascript code. You can block the javascript, which is among other things what an ad blocker does, and the listings will go away, but that is not causing your layout to be crunched together.
The right navigation menu does that. The standard vbulletin theme is table based instead of css based, so you cannot make it smaller, the page will simply show a horizontal scroll bar. The right menu bar is simply not considered to be an ad. I am guessing the people that have this problem are using 1024x768 monitor resolution. (still the most widely used)
I see it too when I switch back to that res.
The only thing that would fix your issue is getting a wider monitor with more horizontal resolution or changing the resolution on your current monitor to a higher setting, which will make everything look smaller.
There are some FF plugins around that allow you to tinker with the html on a site and tweak it to your liking, but that's probably more hassle than it's worth.
HtH