The honest answer on in-house versus agency, from someone who sells the agency. Founders ask me this constantly, usually hoping I'll just pitch. Here's the actual math.
When in-house works
Hiring in-house works when you're big enough that a full-time salary is a small share of your Amazon P&L, when someone senior can manage that person, and when your channel is complex enough to fill their week.
It fails when a founder hires one generalist and expects PPC, listings, inventory, creative, and case escalation from a single person. That role doesn't exist. You end up with someone competent at one of those five things and drowning in the rest.
When an agency works
An agency works when you need a bench instead of a body, when you want people who see 20 accounts and know what's happening category-wide, and when you'd rather buy judgment than train it.
It fails when you buy the cheap version, get a junior with 25 accounts, and pay a retainer for a monthly report.
My real rule of thumb
Under roughly $2M a year on Amazon, an agency is almost always better value than a hire. Over $10M, you want both — someone in-house who owns the channel strategically, and specialists who execute.
The mistake is not picking wrong. It's picking based on price. A $3K agency and an $8K agency are not the same service with different margins. The difference is who touches your account and how often.
I'd rather a brand hire well in-house than buy a bad agency. Which are you actually solving for — a missing body or missing judgment?