I want to sell to advertisers directly rather than going through a network.
That's fine, but be aware that you'll probably make less money that way. You'll have to do a lot more work in identifying your prospects, making contact with them, and convincing them to advertise with you. Once you get past that hurdle, you've got to handle the credit control, billings and payments.
If you hire someone to do all that for you, as you suggest, you will have pay that person to do it. It's obviously going to be time-consuming, which means that it will expensive.
By contrast, if you use an existing advertising network like Google AdSense, you don't have any selling of any kind, and the payment is guaranteed. It's true that the income per impression will be lower, but that's not the point. It's the bottom line that counts.
But to go back to your original question. If you definitely want to go for a do-it-yourself approach, then you've got to come up with some arrangement that will benefit both yourself and the person you employ to manage and sell the ads. The best advice I can give you is to discuss the options with the person who you choose to do this work. Ask them, for example, if they prefer an hourly rate or payment by results. Then start negotiating from there.