The reason you seldom find them in charity shops is that they tend to be snapped up. They have quite a cult following. There is a specialist publishing company, Girls Gone By, which is gradually republishing the titles, including bits which have been cut out of them by paperback editions over the years. They do so in quite an academic way, examining and correcting errors in the original editions! They include covers and illustrations from previous editions.
I loved the books as a child, and enjoy re-reading them now. They are an insight into the world of the early 20th century, with charmingly old-fashioned approaches to manners and speech, management of illness and healthy living, and fashions. The messages of attitudes for young girls were good, I think. No television, computers or mobile phones of course. A lot of emphasis on working hard at school subjects , but plenty of making your own fun. They had an element of history, in the schools reaction to World War 2. The settings, in Austria, Switzerland and also Guernsey and West Wales, were charming
Sadly, my daughter didn't share my love of the books, of which I have amassed a couple of bookshelves-full, having read them as a child from the library like fairfraise. Who knows, I might have a granddaughter to read them to one day! Boys didn't figure much, except as the children and brothers of the main characters - and the eventual husbands, who were all, it seemed, terribly important professionals!!