I had a Co-Founder before and I know what that kind of individuals are capable and how your focus becomes just build/ship and his sell/market it.

It feels good, so good luck finding someone.

A good marketing partner will tell you what to build. 100% of success is building the right thing and being able to reach the market.

Unfortunately, of course, there are lots of bad marketing folk out there. I've lost count of the "if only it could do x, I could sell millions" type promises I've heard. If you don't understand marketing metrics you'll waste more times.

To be clear, a good marketing person is necessary. But you need the skill set to evaluate good in this context. Or you need to be lucky.