Ah, yes, e-bay insertion fees hurt -_-;;

Here's my general opinion, which could be wrong,
but here it goes:

In general, when an item doesn't sell, I'll relist
it maybe one more time, but if it doesn't sell by
the second time, I'll just hold onto it.  Just
wait til e-bay has an insertion fee sale, and then
try selling it again.  It helps to use Turbo
Lister, as you can easily relist it.  

Before listing expensive cels, try to check if
similar pieces are selling  (completed item search
on e-bay).  Also, you can send messages to other
e-bay members advertising your auction (look for
people bidding on similar items, who also bid in
your asking price range.)

Other than that, try selling it on these boards &
related mailing lists, and if you have some
webspace, create a webpage to list your cels--
that'll help simplify your selling posts.

(The following is for mailorder, but it may
slightly relate to selling cels on e-bay)
The ranking of the best time to sell, from best to
worst:
1. January
2. February
3. October
4. August (this month)
5. November
6. September
7. December
8. July
9. April
10. May
11. March
12. June

There are no real secrets to selling on e-bay... I
wish there were...