Chained Payments with bitcoin like Paypal Adaptive Payments?
There's nothing out there that does this but it would not be a hard thing to create.  It's a relatively simple process:


set up payment for xBTC with a specific address to which to send the payment
when the payment arrives create a new transaction that sends some of the coin to an address owned by you and some on to the store owner's address


All of the complexity will be in the business rules: what's the percentage split, how long 'til you pass the coin along (in the case of shipping physical items you need to worry about a purchaser claiming against you for non-delivery), what happens if items are sourced in USD or other currency and translation is required, etc.