When testing the card checkout features on sandbox its advisable to use test card numbers  The sandbox pretty much accepts anything that passes the Luhn Algorithm test (See here).

You can generate card numbers using external sites like these:

http://www.fakenamegenerator.com/

http://www.getcreditcardnumbers.com/

http://saijogeorge.com/dummy-credit-card-generator/

Or you can generate them within PayPal using this guide below:

1. Login to developer.paypal.com and click "Dashboard"
2. Under the sandbox heading click "Accounts", followed by "Create Account"
3. When creating the account make sure to Add a credit card. 
4. Once created you can view the account profile, click "Funding" to see the test card number: