AsiaOne via The Straits Times reported that Mr Sun, a customer service assistant at Times Bookstores at Plaza Singapura , said the book is "moving a bit slow", and as at Thursday afternoon, only five copies have been sold.
The 24-year-old attributed the slow sales to Thursday being "a working day".
James' new book was released at midnight across the world to mark Christian Grey's birthday.
Mr Sun said he is confident sales will pick up over the weekend.
The store sold "over 200 copies" of the trilogy in "about 20 days" before the premiere of the movie on Feb 12 this year.
Books Kinokuniya at Takashimaya Shopping Centre has seen brisk sales of the book however.
Ms Elyana Amir, 20, a part-time bookstore assistant, said customers called the store to reserve copies close to two weeks prior to June 18.
She said: "Shelves are getting empty and we are constantly topping up."
For Grey fan Ms Vivien Quek - who read all three books online - the hype of the novel has "died down since the movie was out".
The 32-year-old said: "We know the ending and reading Grey is just an add-on."
Ms Quek said that she would also read the latest book online.
The trilogy sold more than 100 million copies worldwide, was translated into 52 languages and the film adaptation transformed Jaime Dornan into one of Hollywood's hottest men with the sales of US$500 million (S$664.7 million) at the global box office.