The issue is there is a difference of goods being carried that are personal goods, and goods that are for commercial purposes. You have a duty free allowance for goods being carried for personal usage and consumption. There is NO duty free allowance for goods being carried for commercial purposes, and is the discretion of the customs officer if GST & Duty will be collected, and charged, and would say that if you cannot prove that you have a GST registered company in the country you are entering, this discretion would not be extended to you.
Also goods over a certain value, have to have a formal customs entry filed, and the goods must be surrendered at the boarder until this formal entry is made.
There is a reason why on every entry form, there is one line talking about your personaly duty free allowance, and a second line asking if you are carrying "ANY" commercial goods or samples - again simply as there is no duty free allowance in any country I know of, for commercial goods carried on ones person into the country when arriving via Air.
Into Australia you can "post" commercial goods under $1,000 AUD and they will come in without GST or duty, however get the same commercial goods and bring under you arm when you fly, and you will be charged GST & Duty at the airport. Also the threshold before a formal entry has to be made is not $1,000 either - I think it is $800, but could be as low as $250.
I carry commercial goods each time I fly, and it does subsidize my travel to a certain degree, however the issue as I see it is we are all so busy, that most people who want something sent, what it sent today - and are happy to pay the going rate to send it the normal way that you waste hours of time trying to tee up someone else to carry goods for you. Spending $100 in time, to save $20 in freight.
As also pointed out, you have the issue of trust in what you are given to send, is not illegal.
