On the subject of sales, I think its a mix of things that makes it sell as well as it is.
Part Gail Simone fans, who will buy whatever she is writing.
Barbara Gordon as Batgirl fans, who's been waiting 20 years for it.
And Barbara Gordon fans in general.
I say this mainly because I dont remember any of those factors either alone or in pairs being very successful commercially, and atm I dont really see the book being considered a critical darling (unlike say, Saga or Azzarello's WW).
But it seems whatever it is that makes this book sell is working.