“User accounts” or “users account”
Which is correct: book market or books market? Similarly, is it pirate bay or pirates bay?
'Book Market' is the correct definition for a market where books are sold. It could be one certain book's market, however a market is not commonly in possession of one book, so no.
Same with 'Pirate Bay' (that is the correct definition), it could be a bay that is in possession of pirates, and in that case it would be "Pirates' Bay" with an apostrophe at the end of pirates (plural possession), but as a name it would always be 'Pirate Bay'.