A few of the clients we work with:
Yes its possible, but its extremely time intensive, plus you would have to learn it all your self. Adding marketing to the "to do list" is not the best option...
Finding good people isn't easy. Training them and onboarding staff is costly and slow. Even with the perfect employee, your dependent on them for new clients...
Sure, this risk can pay off, but truth be told agencys care about there business not your's. Thats why they wont offer you a guarantee...