Because for several years now sellers have only been able to leave positive feedback for buyers.
Use the Unpaid Item Dispute (UID) case and if the buyer still does not pay they will have a strike against their account for the next 12-months.
Then make sure you have your blocks in place to block users with 2 or more strikes in whatever time you wish to select (we use 12-months as this is more stringent).
This will reduce the repeat offenders from bothering you.