Beyond that, you can learn a ton to start just by reading things like the Google analytics or AdWords/FB help docs. All of them.
Everything else is all about experience and learning what works and what doesn't and why. There's a lot of good stuff to read out there if you can separate the good stuff from the "gurus" peddling their latest course. Industry sites like Marketingland, ClickZ, industry subreddits like /r/ppc and /r/analytics, etc.
Can you also recommend any resources that helped you become a better seller and marketer?
Books, lectures, etc? Thanks!
Another example would be outright ordering products, warehousing, selling, and fulfilling the orders. That is riskier, but the rewards are much higher.
As far as how to become a better marketer, sadly, you just have to do it. With practice, you will find a style that works for you and that you're comfortable with.
Point is that you dont have to sell onlinem or setup a website, store or anything to start learning. My first experience was selling stuff from a catalog. I was 14.
For many technical people it's quite exhausting to preach what they think is nonsense.
Think of who and how people benefit from your project.