Commercial software is made by full-time Software Engineers while free software is created by many volunteers. Most people think that paid software is always better but there are such examples that volunteer spirit has surpassed successful commercial projects. For example, currently, “Lichess.org” is way more popular and more functional than “Chess.com”.
Free classifieds scripts can be freely downloaded from a developer’s site but in fact, free version of commercial scripts are generally better than free script (For instance, Vanilla Forum vs PHPBB).
Unfortunately, In some cases, free version of commercial scripts are actual traps that aim to force you to buy paid version of the scripts. There is always a possibility that they will stop improving their script and if they are not improving their product it means the script will be outdated and you will need another script which is not very good for you. You have to keep in mind whether your own programming skills are suitable for the project as well.
But some free scripts are actually selling some other related products. They hope that you will come back to purchase plugins and themes.
In summary, I believe if you are not really competitive, starting with free software is better. Also, it is good to see passion is beyond monetary value.