It is up to you I guess, but you could cancel the auction, relist the 2 other books on ebay and tell her you have decided to list the wanted book on gumtree instead.

Even show her the link.

That way she can call around to your house and pick up the book for cash, no fees for you. That is how gumtree works and it is owned by ebay anyway.

There is a slight risk to you in that gumtree buyers are fickle and suit themselves if they turn up or not and invariably try to beat you down.

But it sounds as if she is keen so it would probably work out.