Web application's payment process

We are a new venture preparing to release a new product into the web market. At this time we are looking for a coded API incorporating one of the 2 business flows we are considering (the decision will be made by you). Our application design stipulates the facilitation of payment between a buyer and seller while we, as the "middle man" levy fees to be charged to the seller. The 2 designs we have are 1) the sequential flow: where buyer passes money to our accounts and then we pass money to the seller after subtracting our fee or 2) an "eBay" style where buyer and seller transact directly and we monitor this transaction and its outcome and subsequently bill the seller based on our records. NOTE: The goods being passed from seller to buyer could be of an adult nature and therefore all 3rd party gateways must allow this in their terms and conditions (i.e. PayPal does not allow this). A list of constraints/deliverables: - Target delivery date is 14 - 21 days. - You must present a HLD of the proposed plan and the decision matrix of why you chose the payment gateways. - We, as the middle man, must be able to monitor all steps of the transaction and be issued a report at the end of it detailing the transaction fully. - Expected transaction cost per unit will be $1.50 with an expected 150,000 processed per month - this means we must minimize the fees 3rd party fees charged to us - in the eBay model this would only be when we bill the seller (i.e. batch billing), for the sequential model this would be from buyer to us and from us to seller. - This must be written in .NET v2 (VB.NET preferred) code as an extensible API using XML as the data transport mechanism (web services ready). - The system must be scalable so minimal stateful objects must be used. - The API interface must be approved before "connecting the dots" programming is started. - Documentation of the code (must be CLEAN code) and usage for the API. - A six (6) month guarantee of your work with an agreed upon patching/support process. - The code must be tested and proof of testing must be presented. ---------------------------------- This project is the a very small subset of a much larger project and the winning design, and therefore freelancer, will be offered first bid on other aspects of the project.