I think it'd be nice to have a minimum post requirement before starting a thread in the marketplace, after the recent events. I think something like 15 posts or so, not too high, should be good enough to do the job, otherwise, it may just increase spam from the scammers who really want to get in. As for requiring pics with screen names, I don't think its really that necessary, since people may be trying to use the same pics in multiple listings on say Craigslist or Ebay.
Another good system I have seen is buyer/seller feedback built into vBulletin. However, I don't think that'd work too well here due to the relatively low volume of transactions per member.