Skype supports Javacript code buttons, such as this:
<script type="text/javascript" src="http://download.skype.com/share/skypebuttons/js/
skypeCheck.js"></script>
<a href="skype:tkeating?call"><img src="http://mystatus.skype.com/bigclassic/tkeating" style="border: none;" width="182" height="44" alt="My status" /></a>
Unfortunately, eBay doesn't allow Javascript to be embedded within auctions. Actually that is a good thing since someone could embed a virus into an auction.
Fortunately, Skype also supports direct image links to display your presence. So all you need to do is put this piece of code in your auction and change the bolded part to your Skype username.
<a href="skype:tkeating?call"><img src="http://mystatus.skype.com/bigclassic/tkeating" style="border: none;" width="182" height="44" alt="My status" /></a>
So I decided to create the very first eBay auction integrated with a Skype button. (Link to Auction). Of course, I'm sure eBay is looking to implement more advanced features and integration other than a simple Skype presence button. Wonder if they'll charge extra fee to list have a Skype button? They charge an extra fee for just about everything else.
Update (2 min after posting): For some reason the Skype button in Firefox shows a presence message of "I'm not telling", but the presence functionality works just fine in Internet Explorer. Strange...
Screenshot of my auction: (in case eBay takes it down)