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In the beginning of the movie, shortly after arriving back aboard the Dark Aster, Korath says to Ronan:
We have discovered that Quill has an agreement to retrieve the orb for an intermediary known as the Broker.
How did Korath know that? We saw literally the entire interaction between him and Quill—he figured out that Quill was affiliated with the Ravagers but that was it.
There are a few possibilities here;
Although the timelines don't tally, it's certainly possible that Bereet blabbed as soon as she arrived back on Xandar. This assumes that Quill didn't go straight to the Broker's shop and that there's a gap that we're not seeing.
For me, the most likely option is that one of Yondu's Ravager Clan buddies gave away/sold the information. They don't seem especially savoury people and quite a few of them really seem to hate Peter, even before he double-crossed them. They'd be more than happy to see him dead.
There's an interesting addition to Yondu and Peter's video-call that appears in film's junior novelisation (based on an earlier version of the script), but that doesn't appear in the finished film.
“You know, boss, I feel bad about this… but I’m not going to tell you that.” Yondu’s face twisted in anger. “I slaved this putting together a deal with the Broker for the Orb!”
It's quite plausible that the implication is that video-comms aren't especially secure and that Ronan's men intercepted it in transit. When they updated the scene (to remove mention of the Broker), they likely didn't bother to update the next scene where Ronan magically knows Peter's destination.
