There are many free classifieds script out there but not many classifieds scripts are free.
I have personally tried almost everything I got my hands on.
I can tell you this. I have used many free classifieds script that are free, out of curiosity and wanted to be as thrifty as possible.
They are really, really, not reliable at all, has/have bugs/flaws/weakness/security issues along the way (you will regret discovering it becoz you lose time uploading and downloading, editing, trouble shooting, worrying, etc.), these so-called free scripts cause high consumption of cpu resources, etc.
You will be guaranteed source codes if you purchase them (and with discounts too)for example from
http://www.free-classified-script.blogspot.com/ Free Classified Script
http://www.free-classifieds-script.blogspot.com/ Free Classifieds Script
All the best.
Cheers.