They wished to announce their existence ron and hopefully scare the order into surrendering.
Given the complete lack of survivors you've implied, does it matter if the trade order surrenders (not to mention the lack of a surrender order)? They're coming in, sucking resources right off the planets, and moving on. Not a lot of room there, unless they're interested in "trading"...
Make it so that the transmitter section is more like...
"Sir, we're getting a repeating com signal from an bouy of some kind, on standard com channels."
"Whats it saying?"
The rating entered a command, and an alien voice repeated in crisp, flawless tradese: "We are the Vasari. We
will harvest your resources. Surrender them, or die as these fools did."
You get a
lot there. First, theres the fact that the Vasari have
clearly prepared themselves -- its in the
traders standard communication channels, in the
traders language, which means these guys have been studying and scouting for a while -- that should be pretty scary since the traders haven't had a chance to learn anything in return yet. Second, you get the name, and the intent: we need your resources, and we
will have them.