- The announcement is available to both the public and the guard; it's unlikely that dangerous criminals would conduct business through a guild.
-- That's not entirely true. They didn't actually verify the identity of the person who posted the announcement; he might, on the contrary, be too confident in his ability to hide, even if someone remotely resembling a guard approaches him, perhaps even by the way he walks. If this confidence is also justified, then this is an especially dangerous individual.
-- Or it could be an operation orchestrated by the guard to catch all sorts of inept assassins. They'll tell you to kill someone, you'll agree, and the next moment you'll be in handcuffs. It'll be fun, won't it? Tell me, Kalin, what do you need the money for now? Are you short on funds for enchanting?