Is it permissible for an employee who purchases spare parts for his company at cost price to take the market selling price from the company and keep the difference, and is the money he previously took in this manner unlawful?

Your friend is an agent for his company in purchasing parts, and he is not allowed to take the price difference for himself, even if he buys for less than the market price. He is obliged to return what he previously took to his workplace, unless he is authorized to benefit from it.