I would really enjoy to see - what prices you paid for your goods from trader - because often you got tons of those and you forget price you paid but overall trading in Bannerlord is gr8 fun for me.
It required some experience - but thanks to that, that it is a bit challenging, its not so boring and require from you some thoughts and not just simple buy in town A look on tooltip so we go to town B sell and again to town A etc - grind. Now you have to risk a bit, do those rumors are correct, Do I remember price correctly, or the margin is to small to risk keeping those good in cargo for few weeks before you'll find occasion to sell it without loss
You can use those tooltips but after one or two games playing as trader - you basically know if those good are worth buying. Well you got your "persistent" experience tells you if you can sell them with profit.
If you want to earn as trader just invest when creating character some points to it. It's like you would expect that without inserting any point in 1h weapons you can hit perfectly. It's a skill like any other in game.
Usually I buy when I see that price is low for particular good and then like caravan from town to town looking for occasion to sell but also buying whenever I see good price. Also visiting villages to buy good it's good way to obtain goods on constant price usually lower then in town but not always.