Today I tried to upgrade Ssh Shell Pro license to core shell. After I install Core Shell I was not notified that I have one year free license. I tried to buy license (1 and 4 years) for Core Shell and I got message that I should contact with iTunes support and next SKErrorDomain error 0 error.
Thanks a lot for reporting the issue! The problem is very likely caused by Mac App Store, I'd suggest contact iTunes support as the alert dialog told.
Also, I wrote a how-to explain why this issue might occur: