The main reason for using encrypted buttons instead of plain PayPal button is to make sure no one can look up the HTML source and inject variables and stuff into the form.
There is no risk when you use the WP eStore’s button cause the eStore has post payment verification which checks every details of the payment against the values stored in your product database to make sure the correct amount was paid for the product or service. The following URL explains this verification process in detail:
http://www.tipsandtricks-hq.com/ecommerce/wordpress-ecommerce-knowledgebase-523#payment-verification
How to Fix the Seller Accepts Encrypted Payment Issue
This error is displayed because your PayPal Account Profile is set to only accept payments from “encrypted” buttons.
Do the following to turn off this option:
1. Log in to your PayPal Account
2. Click “Profile” link
3. Under “Selling Preferences”, select “Website Payment Preferences”
4. In the section titled “Encrypted Website Payments” select “Off”
5. Click “Save”