My husband gets all of his income from selling on eBay (part time seller/part-time student), so I'll give you a few things to consider:
1. Is this a professional eBay seller? You can tell by looking at their feedback history/and seeing how many have "s" by them for "seller." Or click "view my other auctions." The reason I ask is this--if she is making her living from eBay, she expects some no-sales, but more than a couple of no-sales can effect a professional eBay seller's livelihood.
2. If she is a professional seller, she knows what it takes to retract a bid--and that you couldn't do it with less than 12 hours. Also, it seems from her original response to you that she was OK with the cancellation...if she demands payment, politely remind her of your prior e-mails, which you would be happy to forward a copy of to her, and that but for her delay in responding, you would have been able to retract in a timely manner.
3. You said the way you see it, she is not out anything. Actually, she is out the listing fee, and the auction commission, which is a percentage of the sale price. She has to pay those to eBay regardless of whether you honor the sale. It may not be more that a couple of bucks in the end, but she is definitely out some money, and if she relists the item, she will have to pay again.
4. Many eBay sellers have disclaimers on their auctions stating that a bid is an express agreement to the auction terms, and not to bid if you are not serious. I would not expect any legal action on this type of clause, unless it is a big ticket item (think houses, rare trading cards, jewelry, etc). However, it is very bad form to engage in recreational bidding, when you have no intent of paying up...and in some cases, it can be considered fraud (like in the case of badge collectors asking their friends to bid up badges so we will have to pay more...ahem.).
5. Since the auction was completed, she has the right to leave you negative feedback for not honoring your bid. Some eBay sellers take this very seriously--my husband does not think twice about cancelling bids for people with numerous negative comments. YOu can rebut the comments, but remember that once they have been made, they cannot be taken back. Also, she can put a non-paying bidder on your ID. If you get 3 of those, you (or your ID rather) will be banned from eBay. This may or not matter to you depending on your eBay habits.
All of that said, if the facts are as you say, I think that if she is a reasonable seller, she will recognize her responsibility and will not demand payment. IN fact, she may be relieved, because you probably got a really good deal, and she was probably hoping to get more (and now she may be able to get more on the street). *Side note: Auctions that end on Saturdays or weekdays do not bring nearly as much as those that end on Sunday-late afternoon....these are the big $ days in most cases because people have been watching the auctions all week, and are waiting to snipe at the last minute--and all time zones can be involved in these!
Let this be a lesson to you!

Don't bid on anymore eBay items unless you intend to follow through with it! (also, on a side note, don't bid on multiple different auctions of the same thing, hoping to win one of them--because you may win them both! It is a pain to the seller you choose to dishonor when you choose between the two).