venting an attic with metal roof

Think I lost my post. We added a metal roof to our house two years ago. Now that we are selling, the inspector said we "must" have some type of ventilation in the attic cause the air is stale in our attic. (We can get from end to end in our attic with some crawling.) I don't know why the roofers didn't tell us this when they installed the roof. They are the ones that installed the roof of the man who "actually makes the metal for roofs" in our area so we thought they knew what they were doing. Is a venting system required to sell? If so, which is the best and cheapest way to go? (The little vent systems we had in roof were covered up when they put on the metal roof.) We heard negative things about gable fan vents with electrical problems later and the snow/water/leaves with ridge vent. I've included a pic of house if that helps.