App store error 100?

Confirmed that the reboot fixes the Error 100.

It should also be noted that you might experience another error dialogue.... that might appear as a blank dialogue but does provide a "retry" button. Once you click on "retry" then you should be good to go.

I have restarted the computer a couple of times and I am still getting the error code 100. Any other fixes?

Start iTunes, do something like update your apps or download a free app, this will prompt you to accept new T&Cs, once this is done, should be OK.

Thank you KirkW. First, I didn't even know I could update my iPhone apps in iTunes, but doing such solved the Error 100 issue. Spot-on advice.

Close the mac store app.
Close iTunes.
Restart the mac store app.

It didn't work for me until I also closed iTunes.

I found that this error occurred when trying to purchase Aperture via the new App Store. I had previously downloaded the trial version via the old Apple store and having it and its installed folders on my hard disk prevented the App store from letting me purchase.

So, if you have a trial version, trash it, and trash the related folders in your user Library and also the Aperture project folder (wherever you saved it), typically in the Pictures folder.

Then try to purchase. Voila!