If you compare os commerce, wordpress and magento, of course magento would be more stable and professional. You will need more powerful hosting for magento, but other than that it's same expenses in future. I saw a private sale solution, developed by Plumrocket company. I think you can find it on their site. Its basically a package of magento extensions and magento private sales theme. You can also extend it with cart reservation, magento invite friends functionality with ability to import address book and magento reward points extension. If you really like Fab.com or Gilt.com you can add some magento social shopping extensions such as live feed plugin, inspiration wall, faves… etc.