MarketPress Strange Behavior

MarketPress is exhibiting strange behavior on a site I'm working on.

Users are required to register and login in order to access the store. Logged in as Admin I have no problem when clicking the 'Add to Cart' button. The cart is populated and it shows in the shopping cart widget (first screenshot).

However, when I repeat the exact same process as a normal logged in user, the shopping cart is NOT populated and I get an 'Out of Stock' notification (second screenshot). This is not even activated in the MarketPress settings.

Does anyone have an idea why this might be happening? I use WordPress 3.2.1 and MarketPress 2.1.6. Membership is handled by S2Member (sorry - I just like it ).