Payment Gateways Sandboxes

PayPal:

Both for buyer and seller
https://developer.paypal.com

Authorize.net

For seller:
http://developer.authorize.net/testaccount/

Google Checkout:

For buyer:
https://sandbox.google.com/checkout
For seller
https://sandbox.google.com/checkout/sell

2Checkout:

http://developers.2checkout.com/