I want to buy a new Mac , but I want to sell my old Mac ?

Depending on your physical location on this planet - Apple does offer a Trade-in Process where you agree to the Term and Conditions and the Value of the Old Computer can be used against the cost of a New Purchase from Apple.

I am located in 🇨🇦 so this following link will be for Canada

Turn the device you have into the one you want.

Hi,

See: What to do before you sell, give away, or trade in your Mac - Apple Support