I have an 8703e with U.S. Cellular that I bought off eBay.
Whenever I go to BlackBerry Internet Service to create a new BIS account it says "An account has already been created with this PIN number"

Is there a way I can delete her old account/bypass this? I'd like to be receiving my own emails.

call the carrier, they can clear the old account. or you can contact the seller to clear their account.