That'd be a good idea but I think it should be disabled in places like the market place, a bump timer maby? Not everyone searches pages for items for sale so maby we can have like a 30 minute or hour bump timer for market place.
i dunno if that would be good, because in a few cases people double post in order to post more links to proggrams or what ever, i think that if you have a post count below 50 than double posting cant be done because by the time that some one gets more than 50 posts they can make good choices...usually..