Expecting a minimum post count before you are allowed to sell on here is a good idea.
This isn't about denying the rest of us the opportunity to buy parts or cars its about protecting the forum and its regular contributors from being conned.
There really isn't any ambiguity, the existing members must be placed ahead of new members.
Yes, some will spam the forum to get their post count up,however, I'm sure their posts will get deleted and they will be banned, simple.
I cannot think of any reason whatsoever why there is a need to put longstanding and/or regular contributors at any risk whatsoever because its deemed that we could be discouraging new members or denying people the chance to purchase cars or parts.
As for 'it wont work' Id think it COULD work, it could certainly have made a difference regarding the ease of access to the huge potential customer base the recent scammers tried to pick pocket, and in once case, succeeded.
90% of us are savvy enough to trawl the net for parts and cars, nobody is likely to be missing out on those bargains of the century because somebody has had to demonstrate that they are here for reasons other than just to sell their car or part.
Honestly, the vast majority who sign up to sell never stay and contribute, they just get a free ad seen by an audience other places could only dream about.
That's got to be a tad immoral if nothing else.