Found some of the Magento extensions are quite sophisiticated and built-ready, while searching for the B2B solutions.
These could help you to come up with a Magento B2B / Marketplace e-commerce platform very quick.
Magento Builder Theme
https://www.magentocommerce.com/magento-connect/frontend-builder-theme.html ($9.00)
B2B / Market place
[Recommended] https://www.magentocommerce.com/magento-connect/marketplace.html (Magento Marketplace Multi Vendor Module US$349.00)
[Membership addon] http://webkul.com/blog/magento-marketplace-membership/ (US$149.00)
[Marketplace Mobile App] https://www.magentocommerce.com/magento-connect/marketplace-mobile-app.html (US$599.00)
author: https://www.magentocommerce.com/magento-connect/developer/webkul
https://www.magentocommerce.com/magento-connect/b2b-suite.html (US$250.00)
https://www.magentocommerce.com/magento-connect/marketplace-2.html (FREE)
Membership
https://www.magentocommerce.com/magento-connect/membership-program-for-magento.html (US$199.00)
https://www.magentocommerce.com/magento-connect/marketplace-membership.html (US$149.00)
https://www.magentocommerce.com/magento-connect/membership-subscription.html (US$149.00)
OR build with Magento recurring profile (membership as a vitual product with recurring payment requirement):
http://docs.magento.com/m1/ce/user_guide/catalog/product-recurring-profile.html
Payment gateway - WestPac and eWay
https://www.magentocommerce.com/magento-connect/fontis-westpac-payway-and-quickgateway.html (FREE) [up to Magento 1.7]
https://www.magentocommerce.com/magento-connect/fontis-eway-au.html (FREE) [eWay transaction fee: 1.6% per transaction]
https://www.magentocommerce.com/magento-connect/eway-rapid-australia.html (FREE)
Vtiger CRM Integration
https://www.magentocommerce.com/magento-connect/vtiger-magento-integration.html (US$299.99)
https://www.magentocommerce.com/magento-connect/vtiger-crm-integration.html (US$149.00)