Does anyone know of any content management system or software that offers the functionality of something similar to equipmentsales.com.au?

I need something where equipment dealers can create their own accounts, submit their own business information and add their products to a site?

It would need to be able to handle about 30 dealers and about 20 products per dealer.

I would prefer a low cost and simple solution like a customized wordpress theme but would understand if Wordpress wasn't the ideal system to build this sort of thing on.

What is the best language for this site to be built on? PHP? ASPX? Other?

Any help or feedback would be greatly appreciated.