why japanese people especially the traders put salt at their door?

It is belived that salt will protect thier home or bussiness from bad spirits. The salt is belived also to block bad vibes that come off of people.

Salt is know as a purifier in many customs.

to ward off evils

To prevent dirty elements from going into the offices or homes..

its a superstision that is usually done in the south and is used to keep evil out of there house,etc

They produce lots of salt.

they run out of sugar