Not to be "anti-trust anyone" guy but I have hadsituations where people have told me that an animal I sold them died 1 or 2 days after the purchase. So I request them to send me a picture of the animal or send it back to me after freezing it. I am sure the guy is asking for his money back. I would ask for 5 images of the animal at different angles.
I dont think the transportation was the issue unless he left it in the sun. The box shouldnt have been air tight even if you sealed the top; unless you taped every corner. When I ship animals I always put them in a burlap bag or pillow case and then in a box or container.
So (in my opinion) you sold them a bad/ half dead lizard in which case he should have noticed and not purchased it. Or it died in some manner that probably wasnt your fault. If the animal was so stressed out that it died then thats not your fault. If it stressed out so bad that parasites took over then it would have taken well over 24 hours to kill it.

Food for thought...
miz
-----
Mizzy: frilldragon.com