Three decades after her death in 1976, Dame Agatha Christie remains one of the best-selling fiction writers of all time. It is estimated that, beginning with the publication in 1920 of her first mystery novel, The Mysterious Affair at Styles, over 2 billion copies of her books, printed in dozens of languages, have been sold around the globe.
It is an indication of her continuing popularity that, on those relatively rare occasions when any hardcover edition of one of her books appears at a yard or estate sale, it is immediately snapped up to take its place in a personal collection, or to be added to a book dealer's stock.
Of course, most second-hand Christie mysteries are generally worth no more than a few dollars each: the real values are in the stories. But recognizing a collectible Christie can be quite profitable. (Some copies are presently commanding prices up to $8,000+ at rare-book auctions and at online marketplaces, but as with any collectible, these prices can and do fluctuate.)
First printings of first editions are always in demand, especially of the earlier works as they had limited press runs. In the case of the aforementioned The Mysterious Affair at Styles, it is believed that just some 2000 copies sold, while Curtain, published in 1975, had an initial printing of 100,000. (See http://www3.isrl.uiuc.edu/~unsworth/courses/bestsellers/search.cgi?title=Curtain.)
Condition, as with all books, has a major effect on value. No one is willing to spend good money on a book that is soiled, torn, musty, scribbled upon, or falling apart. Most collectors are seeking books in at least very good or better condition.
Dust jackets ("dust wrappers" in England) are a BIG plus. A copy with its jacket can easily fetch 2 to 10 times the value of the same copy without its "wrapper." Jackets are often graded separately, and there is a market for original jackets alone!
Finally, don't exclude paperbacks from the market for collectible Christie books. Some first editions of PAN, Penguin, and Pocket Books sell for some quite respectable sums ($50+) depending on condition. (Paperbacks, with their highly acidic paper and glued bindings have the deplorable tendency to yellow, crumble, and shed pages.)
In the collectibles market, knowledge is money, so check out these resources for additional information:
"Agatha Christie: A Reader's Companion," by Vanessa Wagstaf, Stephen Poole
"Agatha Christie A to Z: The Essential Reference to Her Life & Writings," by Dawn B. Sova
"The Complete Christie: An Agatha Christie Encyclopedia," by Matthew Bunson
"The Life and Crimes of Agatha Christie: A Biographical Companion to the Works of Agatha Christie," by Charles Osborne
"The New Bedside, Bathtub & Armchair Companion to Agatha Christie," by Julian Symons (Foreword), Dick Riley (Editor), Pam McAllister (Editor)
The Official Agatha Christie Website. Offers games, articles, links, publishing history of each book, more.
Classic Crime Fiction
Bibliographic list of Christie's UK First Editions
DeliciousDeath - The Agatha Christie Works List
This terrific site offers a detailed Chronological Table in addition to11,800+ book jacket/cover art images!
Firsts' Quick Reference Guide to Grading Books
>From First's Magazine for Book Collectors. Also, terms and storage tips.
PAN Paperback Collectors Website
Hundreds of images of PAN cover art, spanning the PAN catalogue from 1947 to 1966, and including all their Christie publications.