If you have his/her eBay username (e.g. mine is woodland_gnome) and are looking for his/her items currently for sale/auction, you can do an Advanced Search limited to his/her items, but I find it easier to just bring up anyone's list of active listings (e.g. click the "View listings" link next to your feedback score), edit the URL in the address bar of my browser to substitute the target seller's username for yours (or whoever's) and hit enter.
