After you setup your Developer Account, you need to setup your Sandbox Seller and Buyer Accounts.
You would first login to:
https://developer.paypal.com/
Then click on the Applications tab.
Then click on Sandbox Accounts, there you setup both a dummy business account and personal account. You can use the business account to create test item button code and then use the personal account to test the code and the checkout process.
When you create your test item button code, the URL points to:
https://www.sandbox.paypal.com
You can also login in to your dummy business account and personal account anytime by going to:
https://www.sandbox.paypal.com
