Max damage is the maximum damage a weapon class can do.
Max defense is the maximum armor an item has.
I remember reading a post with a list of all of the maximum stats (i think it was on questions and answers forum) but for a quick check I would read a few buy/sell posts with max damage items and check the screenhots... For example I pulled up this one... max damage
You can see that the max damage of an axe is 28, it does not have a picture but a lot of posts will.
Hope this helps.