At what kind of ratio though? Higher level mines are less efficient (less extra res per hour for each resource spent on the upgrade) so if its only 1:1 then you don't want to be having a big iron gap (as trading wood or clay for iron would be inefficient- more efficient to get the iron through iron mine upgrades- indeed ideally you would want iron like 1 behind (as iron costs more) and then trade excess iron for wood/clay)
Also you have to spend the resources on a market and you obviously rely on people selling iron so if there randomly isn't any particularly close to you it could be sad.