Hi,

My advise to you is to separate the cel from the 
glass as far as you could within the box. In case 
the glass breaks, it won't scratch up the cel. 
So, in other word, yes take the cel out of the 
frame. I have learned this from experience. When 
I bought my Ryoko OP cel, it was customly framed 
for me. The cel was not removed from the frame 
during shipping, and the glass broke. I was lucky 
(and thank god for this) that the glass broke 
into big pieces and on the edge only. The cel 
wasn't damaged at all. The box was packaged like 
a baby's carriage, but still, you know how clumsy 
these delivery guys are. If your buyer insist on 
leaving the cel in the frame, then you should 
notify him or her that you won't take 
responsibility for broken frame or damaged cel. 
For your benefit, it's best you take the cel out 
of the frame, wrap the frame nicely, put the cel 
in a USPS envelope, and separate these two with a 
cardboard (for protection and to keep the cel 
flat), and then box them up. This way everything 
is protected and the cel is as far away from the 
glass as possible. Good luck!