It won't have any effect on brokering because the rot is applied when the offer expires, not while it sits in the marketplace.
I still think there will be an effect because that just makes the region lords not want to post until they have a buyer so they no they won't lose extra food. And if they know they have a buyer then they can just accept the buy offer, not post a sell offer. One way it could work for the trader is the trader says there is a buy offer in range of him but not the lord so if the lord posts, the trader will broker the deal but the issue is most the time traders don't have the region lords as contacts if its outside of the trader's realm.
