Building something is a common approach devs take. But there is also the option to just selling something you did not make. Its a shortcut to learning because you can jump right into the action.
One example of selling something you didn't make would be to set up a drop shipping agreement with a factory. Build a website and advertise the products, then collect a portion of the sales.
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.
Go to a local store. Buy a product you use or have used before. Maybe a pair of shoes ,some headphones, a keyboard, mouse, or even a box of cookies. Anything. Then talk to your friends or people you know and tell them about what you are doing. Say how you are learning how to sell and that you are selling whatever you bought. Explain how and why you are doing it. Ask them if they can buy it. Let them know this is a product you have used before and that its good. They will buy it.
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.
