I am having a very odd issue using a Multisite with MarketPress.
I've got this plugin installed, and it's up to date from what I am seeing as well, and it's working in some child sites--but not in all.
The problem I have is when you go through the checkout process once you hit the shopping cart URL it automatically redirects you to the homepage of the site.
Even when you attempt to visit the via the URL directly it'll redirect you to the homepage--so it's not just be "Buy Now" button.
Here is a working example within my network:
Please go through and start an order process, you'll see it goes all the way to PayPal without issue.
This site is on the same network, the same settings, and everything on the API end is fine with PayPal.
Please go through a process here. You'll see when you hit "Buy Now" it'll 301 you to the home URL:
I've talked with my hosting provider and it seems that it's nothing to do with php or apache based on the logs.
If you could please help me locate the issue today that'd be awesome.