Can I identify all iTunes purchases bought with a particular ID Account?
One way to do this is by opening Terminal (/Applications/Utilities/) and entering the following command:

grep -rl username@example.com ~/Music/iTunes/iTunes\ Music


…where username@example.com is the email of the account you purchased the iTunes content with. This will look for references to that name in all of the downloaded files in your iTunes directory. I don't buy a lot of music through the ITMS but this seems to identify the ones that I purchased through a specific account.

There is also this Track Down Purchases application. I have not tried it, but it looks like it would also do what you want.