Hi Daniel,
Last year i helped a friend with his class Ad Project.
Starting on track with your question, if you want to actually upload, test and evaluate some commercial scripts to your server, look here: http://scriptmafia.org/clones/page/6/
also look at the ecommerce section as they often get put in there also.
Of course if you try and you like... support the developer and buy a license.
We found most paid scripts allow you to try their demo... from experience that doesn't give you a real feel for how it works since you play with it for 30 minutes get excited because it's all set up and running but don't experience installing, setting it up it and what it involves to get it live and tweaked yourself. In short you don't know if it is within or beyond your current skill set.
Of the better ones are: Geodesic, Noah's Classifieds and i lister (pricey)
Noah's is reported to be well supported by the developer
Like MiC, I 'm fond of developing nearly any type of site on an open source foundation like wordpress or Joomla/Drupal. For a classified ad site, there are quite stable robust plugins for both platforms to give you the power and filter/search-ability you need. The real estate sites I've made for people handle large complicated databases no problem.
A page of Joomla extensions to investigate are here: http://extensions.joomla.org/extensions/ads-a-affiliates/classified-ads
And in addition to MiC's wordpress suggestion (also good) is the classipresstheme: http://wpclassipress.com/
I haven't tried it myself but many recommend it as being functional, well maintained by the author and search engine friendly. Cost $59
My answer, MiC's and any others that follow should get you started.
Best of luck!