Welcome to ESOUI, Lunaugh!
Addons like Master Merchant and Tamriel Trade Center are huge and complicated projects and a bad choice for trying to break into making addons for ESO. You are better advised to look at our
Getting Started guide on the wiki, do some of the tutorials and build a simple addon first, before you try anything overly ambitious just to get frustrated and quit half-way through.
As for the functions, you can find the arguments and return values
on the wiki or in the ESOUIDocumentation.txt attached to the
API patch notes on the official forum. In order to understand how the game itself uses them, you can read the
source code of the UI over on github.