Is refraining from offering a specific product to a competing merchant, at the request of another merchant to purchase large quantities, considered a sin?

If you are an agent for an entity in distributing its goods, then you must adhere to its conditions. If it permits restricting the offer to some merchants, there is no harm. But if it stipulates generalization, then you must adhere to it, because an agent is a trustee and "Muslims are bound by their conditions." However, if you are a merchant selling the goods for yourself, there is no harm in offering or selling them to whomever you wish, as long as it does not lead to an impermissible monopoly or harm people.