As the title suggests I am in dire need of some input/suggestions regarding which system to use for an upcoming web-project.

My client wants to develope a B2B trade commerce portal. The core functionality of the site must be for businesses to post products that they either want to sell to other businesses or buy from other businesses. So in other words it must be a 2-way trading portal / marketplace for businesses to sell and buy trading goods.

The goal is to have appr. 100000 - 300000 active users and there will be appr. 1-5 million transactions per year.

As a webdesigner I have most experience working with Joomla 2.5 and 3.xx and I am sure that its possible to develope the site on the Joomla platform.

Here are my questions:

Are there other platforms, e.g. ruby on rails, python or ASP.NET, that you would recommend for this project regarding performance, stability, maintainability and future development?

I have found a couple good Joomla templates that design wise would be good to work with and I have also seen some promising auction-extensions on JED that I can maybe use. If you have hands on experience with a similar project in Joomla you are very welcome to recommend a good template and/or component for this project.

As a final note Im enclined to stick with Joomla unless there are serious technical arguments against using this platform concerning performance and reliability.