I saw at the bottom of the inventory page that there's mention of a quick stock tool. How to I get to it and use it?

first you need to have at least one shop. you can make a shop by hovering over the 'commerce' tab and clicking 'your shops'. then you reach quick stock at any time via commerce > your shops > shop name and clicking the little (qs) next to the shop name, or just visit your shop by clicking the shop name itself and then clicking the link at the top titled 'quick stock' to quickly move items to that shop, your vault, wardrobe, and more.