Provide some sort of CC currency ("stored value") that users can load in bigger chunks and then use to purchase items on the marketplace. The point of this idea would mainly be to reduce credit card and paypal transaction fees, if multiple transactions across multiple sellers can be done simply on an internal ledger.
Traditional payment methods such as credit cards and paypal should obviously not be eschewed in favor of this idea. The assumption by vendors should be that the credit card fees will be applied to their orders, but it will be a nice small bonus when users use the CC wallet and the savings get passed on to the sellers. This can be used as a marketing tactic to entice sellers to the CC marketplace.
There may be clever ways to integrate or crossover store credit with this concept as well.
Be aware of any laws that could exist in relation to this concept if implemented in a way that might qualify Crystal Commerce as a money service business.