Moderator Note: We seem to get many people sign up and only post stuff for sale.
In our forum (hifi-forum.de) we had this problem too.
To prevent our members for fake-sales and to prevent us for seller-only accounts, we got following hurdles implemented:
-Users with at last 6 month of registration AND min 100 posts counted could enter the selling place and post an offer.
-Only non-commercial members are allowed to post an offer.
-The buy/wanted place can be used without having at last 100 posts.
Sure, there were some kind of users, which spam as much as they can to reach the "magical 100" - If we take notice about, they could be banned for the marketplace.
Further some areas of our forum won´t count the posts if written there (e.g. when you talk about football in a hifi-forum, you do it in our general talking area and your post-counter won´t be raised).
Additional, if you post and the last post in this thread was also from you, it won´t be counted too.
And, a really advantage for our moderations work:
Only the threadstarter can post in his buy/sell thread.Users which were interested to his offer/wanted, can only contact him via email or private message.
No discussions in the thread, which often ends in confusing/dispute.
Threadstartes can modify their posts in 24h, a new post will be possible after 72h to prevent massive push-ups.
After implementing these rules, we got a much "clearer" and more "seriously" marketplace.