I was visiting with another children's book specialist the other day and we were doing the usual free association book natter, the topic being the books that seem to be most sought by collectors in the juvenile mystery genre. The list below does not cover the series books (Hardy Boys, Nancy Drew, Trixie Belden etc.) and is by no means complete:
The Westing Game by Ellen Raskin. (This book won the Newbery Award and is probably my single most requested mystery title.)
Enid Blyton's Fabulous Five stories, especially from the 1940's and 1950's. Published by Hodder Stoughton. (My dealer friend is from the UK, I'm not sure these were even published in the U.S., though I remember reading them in California in the 60's.)
Elizabeth Honness. Her books were all "Mystery of" Mystery of the Secret Message, Mystery of the Maya Jade etc. Mostly published by Lippincott.
Phyllis Whitney wrote juveniles, including juvenile mysteries, as well as adult mystery/romances. She won the Edgar Allen Poe Award for juvenile mystery in 1961 for Mystery of the Haunted Pool and in 1964 for Mystery of the Hidden Hand. Whitney's juvenile mysteries were published by Westminster Press which sold almost exclusively to libraries, so finding first edition or even early edition of her mysteries that are not library discard is a major feat.
As I said, this is mostly an off the top of my head observation based on personal experience, not on market research. The flaw with working this way is that it tends to be a bit of a self fulfilling prophecy; these are some of the books I loved growing up, so I scout them as a dealer and find collectors who share the same interest.