I'm going to be putting up a couple of cels on 
Ebay too for the first time before the fees go up. 
 General things I can think of.

In the title make sure have anime cel as part of 
it & series name.

Inside the auction, make sure have good scan of 
cel & sketch(if it has one)

In description, describe cel (size, sequence 
number, any problems) and indicate whether there 
is a sketch.  Say if it is production or not 
(fancel, sericel, etc).

If the character on the cel only takes up a small 
portion of cel - do not crop.  Show full cel and 
possibly say painted image measures 3 inches tall 
by 2 inches wide, for example.

Spell out payment options.  I only do money orders 
myself - but you may take paypal, etc.

List additional expenses to be incurred - shipping 
$ and insurance $