The art of the sword and in a wider scope, the art of battle is much like the art of commerce. Samurai of the other clans would never stoop to learn the art of commerce as they consider it to be dishonorable and without glory, but any Mantis would know that zeni are like ashigaru. The more you bring home, the better your tomorrows will be.
It is easy to learn to wield a katana. You start with the bokken and learn the patterns of striking and slashing. You graduate to learning katas and the care required for your blade. You learn the history of your sword, your school, the forms and such. Yet in battle when the screams of death are in your ears; it is the memory of your muscle, the desire to succeed for your clan and Emperor, your worldly desires that drive you. These are the things that guide your hand.
It is in this mixture of the honorable and dishonorable do we find the great contradiction of samurai. To touch the dead is a great sin, yet we take heads as trophies. Blood on the battlefield is inevitable and if we live long enough we must pray for forgiveness in the temples for a lifetime of duty.
It is in the mindset that I compare commerce to kenjutsu. In the market, every item wanted is a battle field. It is part of a larger campaign. If one spends too much money on rice, then less fish may be gained. If one spends too much time procuring sake, then the silk trader has closed his booth and moved on.
One can learn the art of the duel; one can learn the art of the deal. For you see it is in the eyes of your opponent that you will see them wait. You will see when they will strike. This is fine for a duel or a deal.
On the battlefield however, one must learn to find the best manner to victory, one must learn what to spend for that which is to be gained. Such is the same in the market place. Will you trade the pony for a loss, if it puts the buyer at ease so that when he becomes the seller, he thinks you weak? Will you allow seven to be sold for the price of five to encourage the person to buy more than three? It is here that I find the comparisons to be the same.
The last aspect that shows the comparisons to be the same is in one simple truth. All the training and practice in the world does not mean a thing when compared to the application in the field.
Until you have ended the life of another man, either with steel or with copper; you will never know what works and what does not. At least in commerce the learning curve is not as severe.