Manuscript type: Research paper Research aims: This study aims to examine the Facebook-based grey market transactions of female customers in Vietnam. Specifically, this study investigates why (the reasons) and how (the ways) Vietnamese customers purchase grey (unofficial parallel imported) products on Facebook. Design/Methodology/Approach: Semi-structured interviews were conducted with thirteen female customers who had purchased grey products bought in Japan. Content analysis was applied and findings were discussed. Research findings: The Vietnamese unofficial parallel imported products (UPIP) or grey market has transformed itself into an online customer-to-customer (C2C) market with all the characteristics of an e-commerce market, thanks to the development of social media platforms such as Facebook. This market is expanding because it can fulfil the diverse needs of Vietnamese consumers whose incomes are expanding but whose needs are not satisfied. Buyers enter the market after carefully consulting their own experiences and those of their acquaintances. However, due to their lack of experience in