More interesting is what resources or products would be potential candidates for interstellar trade.
No really.
Assuming you're a hyper-evolved super-intelligent post-singularity being, I predict that nothing is going to make you feel better about your surprisingly boring extra-dimensional existence than watching the pathetic little meat sacks on unevolved worlds continually missing opportunities for advancements and obsessively focusing on irrelevant trivia (why it's just like the good old days!). Just insert the galactic laughtrack signal and hilarity ensues, reality television on a galactic scale.
In fact, for all we know, we're already the galaxy's equivalent of Sea Monkeys (http://en.wikipedia.org/wiki/Sea-Monkeys) whilst assuming we were closer to the Sand Kings.
Or as Edmund Kean said on his death bed: "Dying is easy, it's comedy that's hard."
Information has the benefit of weighing nothing and being easily transmissible at the speed of light.
This is also probably true for interplanetary trade, such as with a Mars colony... though return of very precious materials via something inexpensive like gun launch can't be totally ruled out.
Going full sci-fi mode: if something like consciousness uploading were possible, it would also be possible for people to travel interplanetary and interstellar distances at the speed of light as information as long as something at the other end existed to reconstitute them. So one mechanism of interstellar colonization would be to send unmanned drone ships first, then encode ourselves and go.
Matter is everywhere. Information is what it's all about, and information travels at 'c'.
I need to write a book about this some day. It's just so fun to imagine :)
There is the interesting twist that if you could position some sort of device on an interstellar information trade route, you could harvest both sides trades "for free".
So one mechanism of interstellar colonization would be to send unmanned drone ships first, then encode ourselves and go.
Richard Morgan has a bunch of books about something like this, his hero Takeshi Kovakz travels digitally from planet to planet, sometimes waking up in a cheap or worn out body and having to upgrade later. In this case the bodies are "dumb" and need a mind loaded into them. I can't remember if its one of his books where there is a colony ship that grows children from babies, educating and socializing them with robots, which goes horrifically wrong.
But, really, why would we consider colonizing somewhere so iron-deficient, for example, that we need to tow space rocks of the stuff in?