I don't think it's not, just give it a shot with a domain that doesn't have any traffic or minimal and you'll see the number of visitors being updated by your own visits.
Unique visitors - this is unique visitors to your parked page (not necessarily the for-sale listing0.
Visitors to this listing - people who landed on your for sale page either by clicking on the link on a parked page or by finding it in sedo's listings.