Note: This post does not in any way criticize Farah's Guide. Prices have changed since the 12th edition of Farah's Guide was published, and this post covers that topic.
Many sellers value some revised text Nancy Drew books at far too low of prices due to using Farah's Guide to value the books. Some revised text Nancy Drew books, including #17, 24, and 34 in the final cover art versions, are worth significantly more than what Farah's Guide indicates.
Let's start with #24, The Clue in the Old Album, in the 1977 cover art.
Farah assigns the first printing of this art a value of $15. The second and third printings of this art are valued at $5 each.
Back in the summer, an auction for what was either the first or second printing of the 1977 cover art closed at $17.49, which is slightly above the value of the first printing and far above the value of the second printing. I do not know which book was offered.
I sold this book for $19.99. It does not quite match any of the printings in Farah's Guide, so possibly four printings exist with this cover art. Be aware that a book not being listed in Farah's Guide does not make it rare and valuable; rather, the book is simply not yet listed in Farah's Guide.
Regardless, the book I sold for $19.99 would align with one of Farah's $5 printings. It took around one month for the book to sell which is short, considering that I still have some unsold books on Bonanza that have been listed for three years. This tells me that my price of $19.99 could not have been too high.
Farah values the double oval endpapers printings of Old Album at $3 each. I sold this one for $14.99. This book also took only one month to sell.
Quite often, the final cover art of Old Album sells for very low prices on eBay, but this is usually because the sellers place very low Buy It Now prices on the book. Many of these sellers are collectors who use Farah's Guide, so I believe that they are pricing the book too low based on Farah's Guide.
Based on my observations, the $5 Farah's Guide printings for the final cover art of Old Album are actually worth $15 to $20 each. The first printing of this art, in excellent condition, is worth $25 or more.
The 1975 revised text of #34, The Hidden Window Mystery, is another book that is often valued too low.
It is valued at $9 for the first printing of this art, $5 for the other printings with black and white multi endpapers, and $4 for the printings with double oval endpapers.
I sold this book for $6.99, and it took only two weeks to sell. This is one of the $5 Farah's Guide printings, except that the Farah's Guide value is for books in excellent condition. This book is not, yet it sold for above the Farah's Guide value.
Here is another book like the first one, but in better condition. It sold for $9.99 in only 12 days.
Here is a book with double oval endpapers that sold on eBay for $8.99.
Since the books that are not the first printing are selling up towards
$10, we can assume that the actual first printing of this cover art is
worth more towards $15 to $20.
The 1976 cover art of #17, Mystery of the Brass-Bound Trunk, is valued too low in Farah's Guide.
I sold this book for $9.99, and it is valued at $5 in Farah's Guide. Most any printing of this cover art is worth at least $10, and Farah values them at $5.
The first printing of this cover art, which lists to Sky Phantom on the back cover, is valued at $15, but it does not seem to exist. In any case, we can get no credible report of anyone owning one. Anonymous reports do not count. If the Sky Phantom printing does actually exist, the rarity would warrant a value far above $15.
Almost all revised text picture cover printings for most other titles are generally assigned a value of $5 in Farah's Guide for books in excellent condition. Revised text books in excellent condition are selling up towards $10 each, while books that sell for around $5 are ones that have flaws. The possible exceptions are Nancy Drew #1 up to around #10 in the revised text picture cover editions. These books tend to be harder to sell due to how common they are and often have to be priced lower.
Now let's cover the original text Nancy Drew picture cover editions. These books are in high demand currently and are easier to sell than any other type of Nancy Drew book. Most of the original text picture covers were printed during the 1960s, while volumes that were revised later also have many printings from the 1970s. Farah assigns the printings from the 1960s a value of $6.00 and the printings from the 1970s a value of $5. This does not include the first printing picture covers which are assigned higher values, and I am excluding those special printings from this discussion.
I have a lot of trouble keeping a full selection of the original text Nancy Drew picture cover books in stock. Just last week, I replenished my stock, and someone bought most of them the very next day. I have now restocked them again, knowing that many will soon sell once again.
I find that buyers of the original text picture covers are willing to spend at least $10 per book, sometimes $15 per book. Most long-time collectors probably do not value those books that highly, especially since Farah values them at $5 to $6. However, new collectors are clearly willing to pay far more than $5 to $6.
Some of the mid-range original text picture covers are particularly in demand. #24-28 are very hard to keep in supply. I am now pricing average printings of those titles at around $10 even if they are not in high-grade condition. I am moving towards $15 for those titles if in excellent condition, and keep in mind that I am not referring to first printing picture cover editions. This is for the average original text printing of those titles.
#16 with the first Nappi cover art is quite in demand.
I now price it a little above $10 if in excellent condition.
In conclusion, all Nancy Drew picture cover edition books are worth more than the values given in Farah's Guide. Some printings may be just slightly higher while others are much higher.