I just purchased the MarketPress plugin, I'm going to use it with WP Mutilisites.
I'm going to setup a store for each client, each site is supposed to be password protected,
so only the client can visit their site, then they are supposed to be able to put their orders without having to go through the payment steps, because when an order is placed they will recieve an invoice manually.
So basicly they will only have to press the checkout button, and maybe recieve an confirmation email or such.. I don't know about the security of the "Password Protect" plugin though, maybe there is another way to that..
Now I only get a blankpage when I'm supposed to pick the payment method..