Using a bot to log into other people's paypal accounts will cause problems.  You will need more than just the username and password.  If the account has not been used recently, or is logged in from a significantly different IP address, paypal will let you log in, but when you try to send money, you will get asked the security questions.

You might be able to get away with making a bunch of throwaway (unverified) accounts.  If you receive only a small amount of money into each (less than $500) you could get away with it.  You could then pay the money out after waiting 60-90 days to make sure there's no chargebacks.  This will be a lot of work, and some of the accounts will get closed due to suspicious payments, so you'll have to factor these losses into your fee schedule.  You'll also have to be careful of getting your IP addresses banned.