It's good if you're Turkish and are looking for some good and second handed books. I was looking for some second handed English books but there was only one or two good places and even they didn't have a good selection.
One of the good place to purchase calligraphy products.
The Sahaflar Çarşısı market is located in the Bayezid region. It is a market for displaying and selling books and manuscripts. It dates back to the fifteenth century and is one of the oldest markets in Istanbul. The wooden shops were converted to concrete in 1950 and a statue was erected in the middle of the market for “Ibrahim Mfartaqa” Owner of the first printing house in Turkey.
Nice and historic place you can find lots of books with different kind of languages. Even you don't want to get book should visit for the ambiance especially after noon through the evening.
God for a 5 minute walk through if your passing by the sea or want to buy a book.