How do people hire and trust a media buyer?

[Varun]

So I am looking for answers to two of my questions here. I have a lot on my plate right now and I am looking into hiring a media buyer. Hopefully someone from this forum itself. Currently, I have someone who looks after my email marketing setup but for that, I have just provided access to my ESP and created a new user on Voluum so that he can monitor stats. Don't have to share any traffic source details with him

I don't know how to trust someone with my traffic source access though since most traffic sources don't have a multi-user setup. And I don't want to share my login info directly either. Also, for a particular traffic source, I would like to not reveal details of other campaigns that the media buyer will not be managing. Is it just better to create a new traffic source account with no previous data in that case?

Also, what sort of payment terms do people usually hire media buyers at? Profit %? If yes, how much? Or flat?

Any other input would be greatly