Afaik you can submit any game programmed in any code language to Greenlight. (though it will probably be less recieved by the people "voting" on your game if it is a simple language or you use programs such as PRGmaker.)
If you can sell it is up to if you first manage to convince enough people in Greenlight that they want to buy it.
IF you want to intergrate with Steamworks I've been told it's easier if the language is in C++.
For more information about intergrating Steamworks I suggest reading any posts by AusSkiller when you seearch in Greenlight forums for "DLL".

Yeah you should be able to to do it, you'll just need to export the game to a .exe and make sure you package up all the dependencies with it correctly (make sure you check that the Blender license allows you to legally do that though). As Gorlom mentioned if you want to incorporate Steamworks you will need to use C++ to create a wrapper DLL that exposes the Steamworks API functions via the DLL interface (in C++ define the functions with the following prefix: extern "C" __declspec(dllexport) ) and then IIRC Blender uses python so you would load it according to this: http://stackoverflow.com/questions/5253854/python-import-dll

Hope that helps :)