How to set up a PayPal sandbox "Seller" account, a "Buyer" account and an Application Tutorial
Seller account = Business account
Buyer account = Personal account
You probably only need to create one of each in Developer Sandbox Accounts , but there's no limit if your testing requires some more
To obtain the new REST API credentials, create an App in My Apps & Credentials and select the sandbox Business account to use
Legacy APIs: If testing an old integration, you can still get Classic NVP/SOAP credentials for a business account by viewing its "Profile" within your list of sandbox accounts
Just found myself in the same situation. Like you say it seems to be constantly evolving - not even sure if some of the answers here are still up to date.
Walked through it like this:
- Login at developer.paypal.com
- Go to Applications tab
- Click on the Sandbox accounts on the left hand side
- Any sandbox accounts you already created will be listed. Big blue button Create Account to create more
Think of logging in to the sandbox with your real PayPal account as logging into your Sandbox. You do this at http://developer.paypal.com.
Once you are logged into your sandbox in the middle of the screen, click on "continue testing using Sandbox test accounts".
When the next screen comes up you will see a heading of Sandbox Test Accounts and to the far right of that a button to Create Account. Click on this and then you need to create a "Personal" buyer account (your sandbox buyer who you will test with) and a "Business" seller account (your sandbox seller who you will test with).
Set up each one of these as if you were setting up a new Paypal account.
When you have them created fully you will have fictitious bank accounts to work with.
Beyond that, you may wish to check out these tutorials http://www.developphp.com/list_php_video.php where the site goes in-depth with setting up PayPal payments on a website using your own custom, basic shopping cart with a PayPal IPN (instant payment notification). You want to scroll down to the "E-Commerce Website Tutorial...". Lessons 9-20 deal with the cart and accepting PayPal. This may be an older way to do things, but it will help you get a general grounding in working with PayPal IMHO.
You also can visit https://developer.paypal.com/webapps/developer/docs/ At the bottom of the column is a heading "Classic API's" that may provide the documentation you are looking for to move forward.
Good Luck!