What traders can add to the society ?

Broadly, trading is morally neutral, much like many "modern" careers which do little more than process or refine data.

I can think of a localised societal benefit. If you make money trading and spend it in the place you live, you introduce money to the local economy from outside (ie from the capital markets). The same might be said for paying taxes, however reluctant you might be to do that.

On the other hand, many local businesses providing tangible, real services (eg builder, plumber or, (ahem) birthday card shop) don't really bring external money to the local economy, but will just recycle local money.
 
Top