Why not just close the Marketplace for good, and start a form of classified section where (long time) members, preferably just contributors, can post their hardware/software for sale (or WTB and other deals). Members who want to use the classifieds even should have to send in an application form, positively identify themselves to get permission to post in the classified section. A privilege that could be revoked for (suspected) scammers and those who seems to try to run small businesses of these boards (which never was the intention behind the marketplace, right?)
Eligible members get to post an "ad". No thread with lots of bumping and discussion, all communication between seller and potential buyers could be done using PM/IM/mail, but the seller should be able edit their original "ad" to adjust price/shipping etc. and also tag it as "sold" when done.