Because when they do a charge back on ebay or amazon, they will start a dispute. And with that, the seller can show a receipt of the shipment and the charge back will be cancelled again.
Create a marketplace where peoples who need bitcoins will buy products in stores with cash for the peoples who want to sell their bitcoins... So instead of treading directly between cash and bitcoins we will trade between bitcoins and products... Later if the person really want cash he can directly resell the product...