If you are trying to sell products for $0.00 essentially giving the product for free then there are three options for you:
Option 1) Use the manual checkout so the customer can bypass the payment gateway (for example, PayPal).
Payment gateways won't let a $0 transaction to go through so you can instruct your customers to use the "Manual Checkout" option. Make sure to check the "Send Product Download Links in the Email" option in the manual checkout settings menu so the customers can get the link as soon as they submit the checkout form.
You can also check the "Redirect to Manual Checkout For Zero Amount Purchase" field which will automatically redirect any $0.00 checkout to the manual/offline checkout.
Option 2) Use a squeeze page form. This is how most people build their email list.
The following URL explains how you can use a squeeze page form to give products for free:
http://www.tipsandtricks-hq.com/ecommerce/how-to-make-a-squeeze-page-using-the-wordpress-estore-plugin-126
Option 3) Use an eStore shortcode to create an encrypted "Download Now" button for a product. This will allow the user to download the item but it won't reveal the true location of the digital item. Refer to the shortcode list documentation page for more details on this shortcode:
http://www.tipsandtricks-hq.com/ecommerce/wp-estore-shortcodes-and-functions-reference-460
