Hi,
Because your listings are actually posted by Auctiva for you when you post using our website, the ip address of the computer you're using is in no way attached to the listing.
Since you're using a public computer, a previous user might have been logged on to eBay's site and forgot to log out. In that case, you should see a 'sign out' link near the previous user's username when you go to eBay's site.
If it still seems like your browser is "linked" to the other user's ebay account, you should ask a fascilitator to help you clear the browser's cookies. Then close all browser windows and open a new one. It should be cleared by then.