I am a developer and I just put in a few months developing my own cart, and let me tell you its no easy task.
I think building out to 500K products isn't really an issue, but i'd imagine the back-end order processing/reporting is where things might take a while to develop/design/explain.
My cart is very streamlined and built for expansion so it might be a good starting point for you guys depending on where you want to go? Do you have a rough spec?
My cart is built on ASP.NET MVC (no more ugly web control output and viewstate!), IIS7, Sql server (or any db really as I used nHibernate), and jQuery for the UI.
Site search uses Solr so you get search faceting also.