If the brand has a Brand Registry, ask them to add you as an administrator. This way, you can remove the unauthorized sellers. Additionally, consider implementing transparency codes, which will help you sell exclusively.
Brand need to register trademark on Amazon and then authorise you as approved seller and give the control over listings - you will be able to issue IP claims etc.
Ask them to register on Amazon and get their brand enrolled in Amazon Brand Registry. Once that’s done, they’ll be able to authorize you to sell their products under their name. The process involves just a few simple steps and shouldn’t take more than a couple of days.
Without that, you will be only another seller on the listing even having direct authorisation.
I have just had a look and brand is already registered on Amazon, but a sea of sellers are selling it, what should indo now is there a specific document or letter from brand I have to submit to Amazon?
So if they have the brand registered on Amazon I should ask them too authorize through their Amazon brand registry?