What I did was e-mail the sellers who were copying
my cels and asked them nicely to remove the
auction and/or picture from their website (or,
avaters as I have seen some which still had my
watermark on it!!).  Point out to them that they
are breaking the copyright law and its a violation
of your property rights (the same reason a museum
does not allow you to take pictures inside).

Usually, they are nice enough to take down the
auctions, but if not, you can try and file a
complaint with Ebay.  Keep on haggling them about
that as they tend to be REALLY slow about it.