I had the same problem:
I rebooted into safe mode (press start button, wait until boot-up sound, press and hold shift key until Mac symbol and loading symbols show, then release). Then loaded App store, downloaded a free app, which worked fine. Rebooted normally, loaded App store and it seems to have cured the problem, successfully downloaded Mountain Lion. I left everything, including Virus Barrier X6 running with no problems.
tjmarine6492 remedy worked for me. Downloading now.
"Ok, I was having the same issue, tried everything and nothing worked. I finally stumbled upon a post from ifixit or mac fixit and they said:
"Another approach that is working for people is to clear out caches and other temporary files used by the App Store. To do this, open the Terminal utility (in the /Applications/Utilities/ folder) and run the following command:
A Finder window will appear that shows a number of folders. Locate and remove the folder called "com.apple.appstore," and then try launching the store and purchasing Mountain Lion again."
I tried this and bam! It started downloading with no issues. All I did was remove the folder to the trash and then emptied the trash to make sure the file was NOT present anymore. I hope this helps."
Here's the clear answer.
If the customer has Intego VirusBarrier X6 installed
1. Open /Applications/VirusBarrier X6.
2. Click on the "Surf" button.
3. Click on the "Ad Banner Filter" tab.
4. Click "OFF" next to "Banner advertisement filter".
5. Start the computer in Safe Boot, then restart the computer into normal mode.
If the customer does not have Intego Virus Barrier installed
1. Verify that the date and time is correct on the computer.
2. Safe Boot once, then restart normally, then try downloading again.
3. Try with a different user account if the issue persists
If the issue persists
1. Open Terminal.
3. In the Finder window that opens, drag the "com.app.appstore" directory to the Trash.
4. Open the App Store and download Mountain Lion.