I'm sorry, but I feel like it's fair game on ebay, first come first served. if she wanted to make the auction only for you, she could have made it a private auction with you as the only allowed bidder. I have a feeling if you ask her she may say "sorry, the other buyer got to it first", she just wants to sell her stuff and she probably knows you have no recourse (neg feedback etc)
Ebay no longer allows private auctions with select allowed bidders. It's part of their new policy.
I'm sorry this happened to you chloehandbags =( It sucks that you spent all this time trying to get this bag and someone else who didn't do a thing benefited from your negotiations.