I found this thread by searching Google for "Unable to Purchase", I have read all 15 1/2 pages of posts and tried a few of the suggestions with no luck until I tried the removal of the SQL file with iExplorer.
First, along with the "Unable to Purchase" dialog that occasionally popped up. I saw two other symptoms of this problem. One started back in January when I started being prompted for my AppleID password at random times:

I was a bit concerned at first since I thought I might be being Phished. Then I found a pattern. The password prompt was appearing everytime I installed an app from a different device or iTunes and iCloud added the app to the device. So, for example, I download a Universal app in iTunes. Both my iPhone and my iPad would prompt me for my AppleID password.
A little later, I noticed that something was happening with my installed apps. After waking the device, I would see some icons grey out with progress bar spaces and titles of "Waiting..." under them. A second later, the icons would reappear (still showing the progress bar and "Waiting..." text), followed a few seconds later by the icons fully restoring one by one (very quickly):

When this happened, my iPhone and iPad would also freeze for a short period of time. This started pretty much every time I woke the device up after a couple of minutes of not being used.
It wasn't until a little later that the "Unable to Purchase" pop up appeared when I was updating my device or purchasing a new app.
For some reason, I can't find the "Media" folder on my iPhone using iExplorer (it's jailbroken), so I used the Jailbreak app iFile to located the Downloads folder. I deleted the "downloads.28.sqlitedb" file, rebooted the iPhone and waited a few minutes. When I woke the iPhone up, I didn't see the greying icon effect and didn't see any freezing either.
I then used iExplorer to remove the same file from my iPad.
It's been about 12 hours since I did this and I haven't seen any freezing or greyed icons since. After installing apps from iTunes, I wasn't prompted for a password on either device and both had the new apps installed.
I'm betting I won't see the "Unable to Purchase" pop up either.
After reading that others have deleted the other folders in the Downloads folder, I'm thinking that I'll do that shortly. I haven't yet for concerns that it would cause problems.
Currently, my "downloads.28.sqlitedb" file is 80KB's where it was over 799KB's when I was seeing this problem.