"The software itself is useful and part of a very big product category, but it's marketed extremely poorly in my opinion "

Yeah, I can see the problem here lol.

Sales is quite simple - you have something that people want. You make those people aware of it and help them understand how it helps them. That's all.

These people are often looking for something and have hacked solutions themselves. They spend a great deal of time, effort, and money on this. That's your sweet spot.

The irony is you're selling something else right now - a job post as sales co-founder. Plenty of people want to start a startup, they just don't have a product to sell. Really good salespeople will want a good product. Selling a bad product often means scamming people.

You have to have the confidence in your product to make it appealing. If you can't sell it to your cofounder, they won't be able to sell it to others.