This IS wrong. Stand by your principles, what she did was theft from Telewest and fraud in taking your money. Surely the whole story means that you have to report her for the sake of other people.
I sell goods on eBay and I should hate to think that anyone could possibly consider that this could happen on my site.
Incidently, I do not believe in charging for postage and am rather saddened by the people who are taken in by a low price and stupidly high postage charge. Also the people who`s "selling price" is next to nothing but plus a ridiculous postage charge. Even if they sell the same item a hundred times and refund the selling price each time, they are still quids in because they do not refund postage.
No doubt your postage charge was quite high but seeing as the item was stole, you should press for it to be repaid.