If the PayPal api doesn't support a "verify" of call, then here's the usual alternative:
- Create a "paypal_ids" database table in your application
- When a PayPal id is entered by someone, check your table to see if you have already validated the id for a previous transaction. If you have, then you're all set. -- You could also keep bad ids in the table, to act as a negative cache.
- To validate a paypal id, just attempt to do a real transaction against the id, where you try to "send" 1 penny to the id. If it works, then you've validated the id. Store your results in your table and go on from there.
This is how paypal validates checking accounts--by doing a small, but real, test transaction.