A free mod for Mount & Blade II: Bannerlord, by Eskalior.

Bannerlord AutoTrader is a trading bot for Bannerlord. It is a utility that will buy and sell goods for you. This is a tool that will allow you to buy and sell items easily. 

If you want to start trading, you can download this tool. Press the “Automatically trade goods” option in the settlements dialog. You will get a trade window with preselected trade options. To finish trading, just press “Done”.

It will buy items that are cheaper than the buy-setting percentage of the average price.

It will buy an amount of items equal to your current money plus everything you sell.