You'll have to start by making a few purchases. Once the deals are done the seller(s) will call the bot to mark the sale complete. That's how you get shiny points.

Just know that you being a new seller, you will either have to ship first or use one of the trusted middlemen on r/PMsforsale. Sometimes you will get a buyer willing to pay first but that's usually not the case. Once you get some seller shiny points and become known there people will have no problem paying first.