Paypal API and Classified Ad Sites

Setting up a classifieds site. Under it’s settings, there is a payment type. I’m going to use paypal.

It asks for paypal API details. I know how to get one. However, I have several questions about it and classified ad sites.

If I have several classified ads sites, you I need seperate API details, one for EACH site? Or can I use the same API for all of them?

This will apply to other paid services plugins and payment gateways that use paypal API’s.