Mac App Store won't let me buy apps: solution

Just tried to buy an app via the Mac App Store and it was absolutely refusing to take my money. Various suggestions I've seen on the web such as clearing caches,resetting them via iTunes advanced preferences, rebooting, retrying, using slight variations of my account name all made no difference whatsoever.

The solution turned out to be manually signing in using the Store menu (manually sign out if you are already signed in). At that point I was allowed to update/verify my billing information and subsequent purchase attempts worked.

In previous attempts, I had not signed in manually, but rather had the App Store do the sign-in after I attempted to purchase.

So needs a little more work...