With the correct payment status of your Apple’s account the buying from AppStore seems to be routine that is not worth the attention. Nevertheless, sometimes, attempts to buy a new app from AppStore may become very annoying. You promised to download the desired app but instead you get "unable to purchase" message. Don't panic, change can come quickly and easily. Just learn some secret for iPad / iPad 2 / iPad 3 or for the newest iPad mini / iPad 4 and follow these simple tips:
If you are purchasing via iTunes
Following the path C:\Documents and Settings\All Users\Application Data\Apple Computer\iTunes\ (for Windows XP) or C:\ProgramData\Apple Computer\iTunes (for Vista/Win7) remove SC Info folder (if there). This will ensure that folder required to authorize your iTunes Store purchases lives safely on you PC.
If you an owner of MAC log in an administrator account, choose Go to Folder from the Go menu in the Finder, type: /Users and click Go. If the Shared folder exists type sudo chmod -R 777 /Users/Shared (on OS X v10.5.8 and earlier) or sudo chmod -R 1777 /Users/Shared (on OS X v10.6 or later). If the Shared folder does not exist type into the Terminal window sudo mkdir -p /Users/Shared/ . After this apply sudo chmod -R 777 /Users/Shared (on OS X v10.5.8 and earlier) or sudo chmod -R 1777 /Users/Shared (on OS X v10.6 or later).
After all manipulations done, restart your computer and try to purchase once more. If "unable to purchase" persists the problem may lay more dipper that is in corrupted downloads.28.sqlitedb file on your tablet. To fix this :