BookInBookOut is a Singapore start-up founded by Tay Wei Kiat in late 2009. In October 2010, it merged with another online textbook marketplace called Buyback Asia which catered to NUS students.

"BookINBookOUT is an online textbook market place targeted at university students which integrates various social networking elements into the process of buying and selling second hand textbooks. BookINBookOUT is used by over 60% of students in SMU, as well as thousands of students from both NTU and NUS. On top of making textbooks affordable to students, BookINBookOUT also integrates strongly with Facebook, resulting in BookINBookOUT being a great socialising platform on top of being the most popular second hand textbook market place for university undergraduates.

BookINBookOUT was founded by me when I first joined SMU in August 2009. I realised that first hand textbooks are expensive, and in most cases, I only need to use them for a single semester before buying a new set of textbooks again. The then existing second hand textbook marketplace in SMU was in the form of a forum; it was very difficult to use, and posting contact information publicly on a forum means opening up the possibility of identity thief. Determined to improve the situation, BookINBookOUT was conceptualised and implemented within a single day. Over the next few weeks, BookINBookOUT slowly grew in popularity, eventually reaching the popularity it has today."

20,000 to 30,000 textbooks were sold from 2009 to the end of 2010.