I had an iTunes issue occur recently that seemed to be caused by one of two recent actions:

1. After following the steps to share music on a single computer with multiple users. iTunes: How to share music between different accounts on a single computer
2. While logged in as myself with my iTunes account I logged into iTunes with another account and authorized some songs.

An example of the error message from iTunes:

There were problems downloading some purchased items.For more information on the items that could not be downloaded, click below.
There was a problem downloading “Example iTunes Item”. You do not have enough access privileges for this operation.Please check that the connection to the network is active and try again.

Here are the steps to correct the issue:

1. Close iTunes
2. Get Info (Cmd-I) from your iTunes Folder
3. In order to use Finder to make changes you will have to click on the lock at the bottom right and enter the credentials of a user with permission to make changes.
4. Verify that the folder is unlocked (the Locked item in the top grouping is not checked). Note this is not the same as clicking the Lock on the previous step.
5. Verify your user name is listed under Sharing & Permissions and you have Read & Write Privileges
6. To add your user name click +, select your user name, and select Read & Write Privileges
7. Select the gear button (bottom of the Info dialog) and select Apply to enclosed items.