Click on User CP
Click Edit Profile
Scroll down to the signature box
Anything you type in here as plain text will show up that way, ie. if I type Jackie then Jackie will show in my signature line. You can also add links here, like to your eBay auctions.
Here is an example of what you would do to add an auction link:
My Auctions (or Suzy's Auctions or eBay Auction whatever you would like it to read)
with the link to YOUR auction list.
Replace: My Auctions (or Suzy's Auctions or eBay Auction whatever you would like it to read)
with what you want.
Here is an example of what you would do to add a link to your website:
My Home Page
Once again, replacing the http://withthelinktoyoursite.com and the My Home Page with what you want the site name to say.
Click update profile.