Forum Moderators: buckworks
No, seriously... Not only will you be spending money on offices, employees, servers, connections, cards, transactions, etc, but you'll also need a fair chunk of money to back up any transactions that occur (imagine having to deal with $500,000 worth of chargebacks that you haven't processed yet), your business partners will likely want some kind of deposit/guarantee, and so on...
As far as legal knowledge: There is an insane amount of legal knowledge required for even the simplest monetary transactions between institutions (ACH/EFT/Wire transfers) and also credit card transactions. My advice: Get a lawyer. ;) ... Oh, also, I Am Not A Lawyer (IANAL). :)
I don't mean to dissuade you, but there's a lot to it, but if Paypal can do it, anyone can. <grin>
MM