I'd start to check how ZOs vanilla code does it in the esoui sources, and not how addons do it.
That's always the best way
(allthough I bet AGS would do it the same way then, if it supports listing, -> Does it? I never saw any feature of AGS that lists items?)
Should be found in esoui/ingame/tradinghouse/keyboard somewhere then:
https://github.com/esoui/esoui/tree/...house/keyboard
This should be the keybind that you use to list an item you have selected:
https://github.com/esoui/esoui/blob/...board.lua#L167
So I guess you'd have to find out how to select an item from your inventory and then use that same function to list/post it.
Here is a check func that checks if you have selected an item to the listSlot
https://github.com/esoui/esoui/blob/...board.lua#L351
self in that context is the object created from ZO_TradingHouseManager:New -> so it's TRADING_HOUSE (see bottom of the lua file).
Please keep in mind that you should not create any automatism addons that spam the server with messages (like try to list all 30 slots at once) as else you might get kicked for mesasge spam and/
or ZOs would even consider changing existing API and making it bad for us all.