Hyphenation is used here to clarify what the modifiers are modifying. Used bookstore is only ever going to be interpreted as a store that sells used books so there’s no need to hyphenate.
Second hand book shop.
Book(used)store
Gently read books.
As opposed to “Read hard and put away wet”?
When in doubt, rephrase:
Purveyor of used books.
It’s an antiquarian bookstore.
Retailer of nearly new publications.
Well, no. All antiquarian bookstores are used bookstores, but not all used bookstores are antiquarian bookstores.
Antiquarian denotes books that are especially old and possibly rare. The Paperback Trader shop in the strip mall doesn’t qualify.
‘Secondhand’ is a word.
‘Bookshop’ is a word.
Secondhand Bookshop.
Previously fingered books.
I think “used-book store” is the most pedantically correct of the bunch, but it looks needlessly fussy and “used bookstore” doesn’t cause confusion.
The other argument pro-“used bookstore” is the way the phrase is normally said, at least in my experience. The stress of “used-book store” and “used bookstore” are different, with the two being semantically parsed and sounding like “[used book] store” and the second as “used [book store]”. In other words “used-book store” to me sounds like there’s a grouping of “used” and “book” in the stress patterns, and “store” gets an equal stress (so almost like / / / in scansion) while “used bookstore”, the “store” gets less emphasis (so something like / / ˘ in scansion.)
It that doesn’t make sense, think of “Bluebook store” (a fictional type of store that sells Bluebooks, like the Kelly Bluebook) vs a “blue bookstore” (a bookstore that is blue.) Despite the fact that a “used-book store” is a store that sells used-books, the rhythm of the phrase as I hear almost always follows the second stress pattern, as if it were a bookstore that is used.
“Used bookstore” is an idiomatic phrase. It doesn’t have to be semantically logical; it’s meaning is understood from context.
The use of compound & hyphenated words in English is solely dependent on common practice & convention – there are no hard-and-fast rules (just like in much of the English language!).
Words commonly used together for one concept progress from separate words to hyphenated phrases to a single compound word. But sometimes stop before going all the way, and sometimes even go backward.
Not since the 1700’s or 1800’s have writers used to-day or to-morrow. And since the 1990’s, electronic mail has been moving to electronic-mail to e-mail to email. But some online grammar/spelling checkers still complain about that.
It’s a moving process, controlled by how many people move with it. Which is why a Google cite count is a semi-authorative answer.
Both to-day and to-morrow were in common use well into the 20th century, though somewhat longer in Britain than in the U.S.
Here’s a major example from the 1920s.