I think we should dig this back up maybe, as there seem to have been some issues appear recently.
I think a time limit/post count limit would be great, then for someone to be allowed to SELL in the MP, you should have to prove feedback from an external source- be it another board, ebay, heatware, etc. I would dare say most people that come here have sold elsewhere and this is not their first rodeo. In today's day and age with the way it appears people are, I would rather it be extra safe, and have less activity than too free, and people being screwed. I also think that the transactions SHOULD take place via some sort of payment system be it paypal or google checkout or something else, simply to give an official aspect to it. This of course could be looked around by a local pickup/CASH transaction.
As far as new feedback goes, Create a subforum in the MP. In that forum all would be allowed would be threads dedicated to feedback from MP transactions. There would be a thread title for each member who has bought or sold in the MP, so it could contain buying and selling feedback. In the case of a dispute about feedback, a moderator could be involved as a mediator and give a third party statement of facts only. Snafu's happen, and mistakes take place, problems arise when someone goes off half cocked about them when the communication was poor to begin with.. just sort of ideas to help reduce issues and keep the market place a fun place for people to practice commerce.
Does that make sense?? Just an idea...