This is a good looking site and it has the potential to be great
I like the logo, nice and shiny! This should always link to the homepage
http://sitemarkets.net/ and not the root of the section you are on. For example when going to the marketplace the logo home link has changed to
http://www.sitemarkets.net/auctions/index.php which is incorrect.
My initial interpretation of the subject of the site is that it's a news website when viewing the homepage, all those columns and lots of text doesnt make the site look like an auction site as your strapline suggests - 'buy and sell digital items'. I would suggest breaking up the page a bit with some graphics and maybe some recent posts from the forum/ recent auction items. Top sellers etc.
Also when viewing the site in Firefox 2 the web marketing and breaking news boxes in the top left are overlapping the content.
Your URLs are not all SEO friendly, there are a lot of ?= URLs. I would recommend tweaking those to include the url subject in the url using your .htaccess file.
Another thing with the navigation and the look of the site - your look/theme should stay constant throughout the site. The main navigation on the homepage should be on every page sitewide.
Also you have a few errors in the html on the homepage -
Empy list (see below in bold):
<ul class="list1" id="pop">
</ul>and you have used id="feat" twice in the page when an id should only ever be used once.
Good job
