Thank you for requesting me!
Yes, there is a market and buyers for Nancy Drew Mystery Stories written by Carolyn Keene.
The values if you have the newer hardcover versions from Penguin Books are about $5.00 to $10.00 per book.
If you have the older books, keeping in mind these have been printed from 1930 to 1975 by different publishers,
the oldest being The Secret of the Old Clock from the 30's worth $30 without dust jacket and $75 with dust jacket in good condition.
The other books from the 30's are in that same range. Those printed in the 1950's are worth about $15 to $20 each.
Some sites that I recommend are Varagesale.com, Etsy.com, Ebay.com Ioffer.com, Krrb.com, and letgo.com.
You might enjoy reading here about the different years, versions and publishers. http://www.series-books.com/nancydrew/values.html
When you do sell, you can mail them via Media Mail so that it would not cost much to ship (just a tip).
Do click the stars after you have read this answer.