To test your button, you can use something called the PayPal sandbox and test your button using fake money.
First create an account at https://developer.paypal.com/developer.
Once you have a developer account, create a sandbox account here:
https://developer.paypal.com/webapps/developer/applications/accounts
Make two account, one to simulate the buyer and the other the seller. You can just makeup email addresses.
Put the email address of the seller account that you just created in the Sandbox Account: field on the Plugin’s settings page. Then turn on sandbox mode using the radio buttons located right below the Sandbox Account field.
Next create a button on your site, and buy it. When you are asked for your PayPal email address, just enter the seller account email and password that you made up.
When you are done testing, please turn off sandbox mode.