PayPal for Digital Goods is a free WordPress plugin that allows you to sell digital goods online using PayPal for digital goods checkout. The checkout process is quick and entirely happens on your site. The visitor never has to leave you site to pay for the item. You can sell any media files from your site using this method.
How to Create a Product Download Box in WordPress
WP iSell plugin allows you to create a product display box that you can show on a WordPress post/page to sell digital download.