What is the ruling on an employee buying a product and selling it for his own account, away from his company, which sells it at a high price?

If there is no condition prohibiting it, and you will perform this work outside of official working hours, then there is no harm in it. However, if there is a condition prohibiting it, or if you will exploit the name of your workplace, or perform the work during official working hours, then it is not permissible except with permission, due to the Almighty's saying: "O you who have believed, fulfill [all] contracts," and the Prophet's (peace be upon him) saying: "Muslims are bound by their stipulations."