That's awful! :[
I really can't believe that she would do that.  

I think that your suspicions are correct as well.
 I think greed must have gotten the better of her
and that she must have gotten a better offer on
the cel after the auction ended and before she
sent it to you.  After this, she was stalling to
make it seem like the MO got lost.  You didn't
happen to sent it with delivery confirmation did
you?  It seem to me like she is now pretty
desperate and is resorting to slandering you.

I would definately do everything within my power
to make this a very unpleasant situation for her
as well *is evil*.  You should contact ebay about
this and make sure that you include the emails and
dates of all the transactions.   Make sure that
you also say that she never told you about selling
the cel to someone else until after you confronted
her about it being in someone else's collection.  

If it actually did get lost in the mail, which I
highly doubt it, it is not your fault!  I am
positive had this happened she would have tried a
little harder to resolve it with you first before
selling it again.  She could have even asked if
you could send another alternate payment and
cancel the MO.

This is really unexcusable..