EDIT: Ok, so the only thing that ever touches that value is a method called "ExecuteUpgradeWorkshop ()" and that's linked to the Clan Finances view model but doesn't seem to be linked to anything in the UI. So I guess at some point they intend it to be something the player chooses to do. No idea what the level is expected to add.
Hardwood when collected in battanian cities can be as low as 11 denari, buy up till 25ish denari, do not sell below 30 denari, in high demand Ive seen it go up to 80 denari each. Flax bought below 10 denari, sell above 14 denari. Iron ore buy below 40 denari, sell above 55 denari. Anything else if you see it low buy it, but dont stock up on it.This is also a good way to get high level gear to sell. (15k for a horse you cant even use until the late game-At that point you can buy them easy.) Fighting the City gangs in back alleys are also a great way to increase renown, get loot, and above all increase relationships with particular lords. 3- Look into marriages for you and your siblings.
Get a few workshops and get the perk for trade rumors from your workshops. Then just go off that. That's the best way I've found so far. This comment was from 8 months ago, they've since updated it so trading horses and mules isn't great. Best way I've seen to level up trading now is the caravan exploit. Mount & Blade II: Bannerlord allows you to develop your settlements into money making machines! But in order to get the most gold flowing through your town, you need to ensure that you have the right infrastructure in place. Setting up workshops is key to doing this, and there are some workshops that are better for specific towns than others.