Rails - Mar 2006 - [Semi-OT] How to minimize transaction fees in escrow like environment

I am currently developing an application in which two users exchange 
goods and money in an escrow like environment.
Buyer has some money and wants an item that Seller has.  Buyer does not 
want to pay Seller until Seller ships the item.

I am unsure of how to best handle the payment part of my application.  
Buyer''s money eventually has to end up at Seller and my company would 
like to be paid for its services.  The way I see things there are 3 
possibilities, each with its own positives and negatives. 


1.  Buyer sends the money to my company.  Seller ships the product and 
my company sends the money minus our commission to Seller.
    This approach involves two transactions and as this will all be 
credit card based(maybe paypal) this means that over 4% of the cash goes 
bye bye for transactions fees.

2.  Seller pays our commission up front and we tell Buyer to send the 
money directly to Seller when Seller gives us proof of shipment.
    This approach only involves one transaction for the consumer and one 
transaction on our commission.  So we make a bit less per sale but the  
Seller makes a bit more and is happier.  We would have to figure out a 
way  to  let Buyer send money to Seller without using an additional 
transaction to go through us.

3.  Buyer sets up some kind of hold charge for Seller.  When Seller pays 
us and provides proof of shipment we remove the hold and Buyer''s cash 
goes directly to Seller.
    This involves one transaction for the consumer and one transaction 
on our commission like option #2 but we get paid at the end instead of 
the beginning.  I don''t really know anything about hold payments and I 
have no idea if it is     possible to setup such a hold for one person 
to another as a third party.


What approach would you smart people recommend?  Does anyone have any 
experience with hold charges or using a paypal like service to transfer 
money between two individuals without raking up a lot of transaction fees?

Thank you,
Matthew Margolis