It's one of the more expensive extensions for Magento but if you are looking for a Group Buy / Purchasing extension for Magento then this extension looks very promising. 

There is another extension similar to it in MagentoConnect but the developers aren't running their own Magento store and you can't purchase it from them. Four days after I contacted them enquiring about the extension, I have still not yet received a reply. For those that are still interested, it is called Groupon Clone and is priced at $399.00 at the time of this writing.

I haven't yet implemented either, though a project I am working on will use one or the other, likely the XinPlaza one because it seems I can actually buy it!