how to protect uploads folder so user cant simply go to the url and download content

Hi, im using the MarketPress to run a ecommerce store store with downloadable images and files, the plugin works great and I have set up paypal and everything, though the user can simply navigate to the uploads folder and view my content and dl it without paying.

How can I stop acces to my uploads folder without it interfering with the plugin? as when the user has paid and receives the dl link, will it stop them?