This part we didn't really innovate - that's how plateforms like elance or vWorker guarantee their payments. Taxes can be an headache, but nothing that hasn't been solved already.
How are you handling chargebacks? What if the service buyer does not honor you escrow arbitration after the process and files for a chargeback? are you going to take the money back from the service provider?
http://en.wikipedia.org/wiki/Escrow