The bulk tax i think used to be for selling above 400 of a single commodity, there used to be a thread about it.
Certain backers (above what i paid for) are exempt from paying.
It’s not supposed to be a tax per-say especially not one to slow trading, it was a way of tackling the ‘Lakon effect’ which is basically anyone in a trading T9 or Conda ruining economies and wrecking faction influence for everyone else.
They fixed up the influence effect and now trading does much less damage and about equal to heavy mission running but short of giving all economies static prices you can’t do much about the damage Lakons do.
The tax is to encourage diversifying so it becomes profitable to run 2 commodities rather than one.