It appears that the website broke sometime mid-November 2016. All selling was turned off, and nobody could buy photos. Customers could create "favorites" lists, but the "buy" button was absent.
I've corrected the problem, and it's possible to buy photos again. I truly apologize for this. I'm not certain how it happened, but I'm in communication with Zenfolio technical support to find the root cause and insure it doesn't happen again.