I would really need to see the ads that you're selling to be able to suggest what method to scale them with... but you hit the nail on the head...You have two options really...building internal sales processes or going through an agency. Usually companies go through an agency first, to get professional handholding in "sales product" development...ie. they help you scale your pricing and ad units and offerings...and get your foot in the door with bigger clients...but you can do a short term contract with most agencies...say 6 months...so that after 6 months you are in a much better spot to handle it on your own and stop paying 30% to an agency...