I tihnk you are a little above your station pal.Furthermore, up to the point of the marker arriving at Stu's house, it is your responsibility. If you chose inadequate postal insurance that does not cover the value of the marker, the blame cannot be placed upon Stu and it is absurd that you are contemplating taking him to court when the blame lies in your hands.
Your respectable friend lets not forget, sold a broken marker as working in the first place and now you are talking about inadequate insurance?
I would also point out that this guy is not on here defending himself either after clearly having been informed of the situation.