Passing a payment from buyer to seller, and taking a transaction fee?

I'm mulling over a business idea, and the part of it that I can't wrap my brain around is the payment processing part of it. Here's a simple example, not really my idea, but an easier example:

I have a consignment shop site, where people post items, and other people buy them. As the site's owner, I'd like a cut of each sale, and want to take 2% of the sale price as a transaction fee.

How would I go about doing that? The only way I know is using something like PayPal, having the original payment go to me, and then I pay the seller (minus my fee), but that seems like it would rack up a lot of PayPal transaction fees.

Can someone clue me in on a better way to do this? It seems like a pretty standard business model. Thanks!

What you are looking for is a marketplace solution. In your example, Paypal would only be able to receive the payment - establishing the connection between the seller and buyer would be on your side - and (depending on location) there are fiduciary responsibilities associated with holding and transferring that money (PCI, etc.)

Depending on the scope of your project, you can consider amazon FPS marketplace, balancedpayments or bancbox. There are others.