I'm really proud of it for a few reasons:

1. It was a response to an observed need. I was getting daily emails from devs asking me about product marketing. I believed that devs who learned marketing could be unstoppable when it comes to launching products.

2. I created it on the side, while working full-time.

3. In its first 3 months it did $28,433 in revenue. This allowed me to go full-time on my own projects this January.

If you build an audience, and earn a good reputation, selling your expertise is a good option.