The Smuggler's Union is the Brethren Trade faction. In order to use the Auction House of a port run by the Brethren, a player must be indifferent or better with the Smuggler's Union.
The faction standing can be raised by using Marks of Trade, doing junk merchant missions or sinking Varyag Pirates on the Open Sea.
The faction standing will be lowered by sinking ships belonging to the Smuggler's Union on the Open Sea.