Hi there,
I'm an experienced developer and have worked with the PayPal API in the past (mostly integrating shopping carts).
I've read your project proposal but I'm a little confused as to what it is you want to accomplish here. To clarify, do you either want to:
a) be able to use your PayPal account to facilitate payments between users, as in, for instance, an e-wallet service, or
b) direct users to log in to their own PayPal accounts to perform these payments?
It's the mention of an API key that throws me here - you do not need to use the API to create custom payment pages (as presumably you would want to theme the PayPal login in the second case).
I'm available immediately if you'd like to please PM me to discuss this further.
Regards,
Andrew