actually I would buy from the person who is selling the cheapest
I believe that in order to run a market that some sort of skill in Personality section should be required. Maybe the higher your personality the more flexible you can make your market..maybe more slots or at a certain level you can set up shop outside of towns.
Also please have designated spots where a market could be set up because i dont want people blocking doors with them...Just have like discolored rectangular areas in towns and have a few scattered in dungeons that are out of the way of harm and out of the way of adventurers (like in the first room of the house dungeons those would be perfect)