He made you wait a month, plus damaged your camera. He should now sit patient as you fully assess all the damage with a professional. As someone else said, cost to restore it + cost of any missing/damaged paperwork. I might even go so far as charging him the $35 shipping since the camera works (you only took it because he claimed it wasnt working). I would also consider your time as something to factor in too.
Once you buy used, unless you receive it not working, damaged or some other issue spotted right away, its "as is" after a reasonable time frame. You are right, none of us are big box and buyers should expect that at these prices, there is some risk on the buyers end and certainly on the sellers end. If you want a solid return policy, pay new prices.