The script is clean, but obviously has several bugs regarding features and process. It looks like a school project, and is perfect for learning php, mysql and basics of database programming, installation etc. The code is nestled, where the UI language is not separated from code. You need to create and integrate location and map-handling for your self. The category management works, but is not connected to the UI, so you need to delete (not just deactivate) the category if you don't want it to display. If a category is free the new listings box will show the listing, but if the listing is not free the the new listings will not show in the new listings box. . . . The same applies for messaging, expiry, approvals, administration ... there are several things to improve. The problem comes as you change code (even translate), the script will not be upgradable.
For basic learning it is great, but for a commercial site it requires a significant work effort which will not be reusable.
I choose not to recommend the script for its purpose, as it lacks many basic classifieds elements. For learning and teaching it is better then buying a book, as you can point out several architectural mistakes and improvement needs.