Back then (in 2004) the complexity merchant of choice was J2EE.
I understand, that merchant is a person, who sells something. I assume that "merchant of choice" is somebody, from who we decide to buy something. So "complexity merchant of choice" is somebody, from who we buy complexity. That doesn't make sense, even in a figurative sense.
J2EE is Java 2 Enterprise Edition and the quote is taken from the text about programming languages and frameworks.