With marketplace, the site admin receives payments via the standard Ubercart payment modules. So if http://drupal.org/project/uc_roboxchange works for you, the site admin can use it to collect payments from customers. The admin must then pay the sellers via whatever means is appropriate for the admin's business, whenever it is appropriate for them to do so.

Support for manually-triggered payment to sellers (the admin clicks to trigger payment) using Paypal Masspay is included. There has been a feature request for additional payment methods, such as Amazon Flexible Payments Service and Paypal Adaptive Payments, but I think right now the only built-in way for the admin to send payments to sellers is via Paypal Masspay or some other business arrangements - in any case not automatic, but controlled by the admin.

Feature request for additional ways to pay sellers: http://drupal.org/node/501826
Payment to sellers is manual instead of automatic by design: http://drupal.org/node/314505