I disagree. After a rash of zero post seller scams, we implemented this rule to reduce the frequency of occurrences. It has worked.
In addition, the classified section isn't strictly open to 'anyone'. It is open to contributing members of the community who want to sell their items to each other.
What we saw over time, due to the popularity of this community for car PC related items, was people who have not contributed to it simply using it to sell electronics. This was accompanied by a sharp increase in the number of scams.
Although Mp3car makes it clear that there is no involvement or obligation on its part if a scam occurs, inevitably members would contact asking for personal information regarding scammers and wanting assistance in the resolution of disputes in the classifieds.
The post limit was imposed in a successful effort to curb the number of instances. It works. Can people spam the forum to try and get a post count to sell in the classifieds? Sure. They can always fake it some way.
I mean this in the most gentle way, as you seem to be quite sincere, but if you are not interested enough in this hobby to contribute to the forums in some way but rather want access to it to sell your wares, perhaps you should look elsewhere. As you noted, there are many, many other places to sell such items than here.