How can I search for MP3 files by the "Purchased By" field?
In Itunes, create a Smart Playlist and name it purchases.  (File Menu, New Playlist... Smart Playlist)

In the search parameters of the smart playlist, Choose 'Purchased' and 'IS True'. That's the closes you'll get to sorting out your purchased items, but it doesn't allow you to sort by apple ID. 



Since iTunes doesn't allow you to search the 'Purchased by' field in filters, you can accomplish this using Tracksift, by dougscripts.com.

I tried, and using applescript doesn't easily lend itself to reveal the 'purchased by' property, from what I can find.  

http://dougscripts.com/apps/tracksift2app.php

This app allows you to Sort tracks into playlists by Apple ID.