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Kinokuniya is kind of the go-to bookshop for Japanese books in Singapore. That being said, because Japanese is such a popular language to learn here, the second hand market for used (and often unmarked) assessment books / textbooks is huge here. Check Carousell (a local app for buying and selling) for very good deals on used books.
If you are buying from Kinokuniya, I've found their local online web store has cheaper prices than the in store prices. If you go above $50, shipping is free as well. If you can wait, sign up for their email newsletter, because occasionally they have 10-15% online discounts for non-members.
There you go! Take note that most if not all listings are local. Not many of them ships oversea.