First be sure to install the toolbar specific to your country - the link above is to the eBay.com toolbar page which has links to each of the local toolbar versions. I don't know why they could not have just one version with an option to choose a country in the preferences.
The toolbar consists mostly of quick links to eBay pages which are handy but nothing to get excited about. The "Bid Alerts' and 'Watch Alerts' are alarms based on items you have bid on or are watching - they are quite a handy way to set an alert. The search box allows you to search eBay without going to the home page first so is a little faster than normal.
In all this could be useful for the alarm and quick search - if you need them and have space for another toolbar on your browser!