i think it is a good idea to offer a central location to do something like this - however, a 70/30 split is quite hefty - especially when the developer can offer their own e-commerce solutions. for example; esellerate (http://www.esellerate.net
) is an e-commerce provider that you can add to your website very easily and they only take 11.9% of the purchase price.
will you be allowing developers to post their game source/info on your site yet allow the visitors to buy from their own stores? (we use esellerate for our non-iphone games)