Off hand, I do know that if I start a service like PayPal, I would need a business bank account setup to run it, probably starting locally to where I live, so that I can begin working with the laws regarding money exchange services. Then I would need to pay the fees required to register a business like that, which would be a bit of an investment to do so. The State I live in, has it's own fees to register, then I have to register it with the Federal Tax people with their associated fees too.
I figure I would need to limit the market initially, so that all tax issues can be worked on.
As far as how PayPal has it, I know they have ties to a number of banks to make it work in more than one country, so that any transfers made in certain countries, would be more or less instant, compared to standard bank transfers across borders.