This is a type of FI (read: a bank). Don't believe in any comment you will read in this thread, you need to seek legal help. (I own a bank/credit card company)
There are state and federal regulations regarding the type of service you want to provide. Don't even try to open a company without completely understanding the business and having extensive legal help.
Those type of companies have very strict guidelines and it is harder to adapt an existing company than trying to open another.
I am afraid, legal help will cost us much more than our limited user base would be able to pay. That's why we don't actually want to register our own company, but rather are looking for a company that provides service similar to what I described.
It's hard to believe that in 2013, when you can do anything online, nobody came up with such a simple service of handing payments and paying taxes for others.
This is more about regulation, not everyone have good intentions. What you could do is to pay with your own money and charge the user later adding your services, this should be legal without going into financials. But again, you got away with the banking regulations but you could have problems with the IRS, this is why you need legal advice.
You can never have an account where others can keep their money, once you receive money in, it should be yours. So, timing is factor a here.
But again, a 30 minutes talk with an advisor is not very expensive. There are offices specialized in opening such firms.