First of all, thank you so much for this video, and kind words, It's really insightful to see how person who is not me plays the game. And I am very well aware, that's not juiciest and most user friendly game, but it was first time I made inventory system and UI like that.
You have to right click stuff to move it to your inventory, and generally, when in doubt or something doesn't work, just right click.
That guy with runes is shopkeeper, you will not fight with him, and to sell just drop something on him from your inventory.
There are timestamps, I hope this will remove any concerns about the game play, but if you have any questions, I will be happy to answer ^-^