Please explain ... what about using blocktrades.

As far as I know, there is no tax liability on any blockchain account because it is not been legalized around the world.

If blocktrades are paying tax, there could be one reason ... to legalize there operation of the conversion of the cryptocurrency into fiat. They could be charging this from you to minimize their expenses.

In my country, I cannot convert local currency into a foreign currency without supplying the prove of its necessity for me, where would I use that money and for what purpose ... Something like this could be the reason for the blocktrades too.

My apologies for my ignorance about blocktrades, I haven't come across their operation and/or in contact with them yet.