I'm a moderator on another forum. Whilst I can moderate across the board, I tend to focus on the Classifieds and general discussion areas.
The rules we have on classifieds are simple, no postings to auction links either for your own stuff or something that you think someone else might be interested in. Any that do are deleted in fairly swift order.
The significant difference is that it is a 'closed' forum and requires a membership fee to be paid to join. That usually puts off those who only join in order to post a classified ad.
Before anyone jumps down my throat, I am not advocating that GJ starts charging to join, I am merely pointing out a deterrent that seems to work.
Whist I would endorse the suggestion on of a number of posts before being granted thread rights to classifieds, this could be achieved quickly in a number of ways & the poster would not have contributed anything towards the forum. I do think this also puts alot more work on the moderators.
I would suggest a simple 'No links to auction sites or other classifieds sites' rule & enforcement of that irrespective of the number of posts.
I also endorse Bull's proposal that "you post your tools for sale here, first, to show the GJ community that you are one of them"
If the current moderators need a hand, I'm happy to volunter my services for the classifieds section. I've got a lot of information from this site for my build project and would like to give something back by way of my time.