My $0.02:
I think you'll should carefully consider what it means to be part of the money flow (instead of maybe just pricing for use of your marketplace - and not get involved in the buyer/seller transaction).
Its really not just a technical consideration - realize that if you do add yourself to that (money/transaction) flow, that you are incurring liability?
- how would you handle returns/refunds?
- disputes between buyer/seller?
- chargebacks?
- if you are using a 3rd party (PayPal or Google Checkout), how would you handle any event that pretty much means a refund/return of monies (the 3rd party makes a policy decision)?
- similar to chargebacks, you should realize that card holders' bank can make their own policy decision as well
So while all the above can be handled technically via "some API that you find", consider what all that means to your business (service operations/fincancial operations/etc).