Brand bidding for resellers

I have been working with a client who sell products that are also distributed through large national retailers, department stores and amazon. I run ppc for their direct website. their cpcs have been rising a lot since distributing through the new partners as a result of higher competition, and achieving a low CPA is getting harder.

My question is, is it common practice for an agreement to be in place to stop the resellers bidding on our brand terms or is that not really the done thing? I havent worked with products that are distributed in multiple places before so im unsure.

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