I'm guess what happens is that the persons account gets hacked. Someone was legitimately selling stuff and shipping it, their account gets hacked by a new person, the new person is a scammer.

My guess is that the seller can go longer before ebay shuts them down if they give a tracking # and maybe ebay can find out if the package doesn't weigh anything, so they throw in items that don't cost them anything. They have a tracking number, the package shipped at 0.5lbs, with 3-5 day shipping it takes ebay a week to figure out that they're now a scammer, whereas if they didn't ship at all or shipped empty packages it might only take 2-3 days.