With no Listener how was the DB getting any jobs at all? I mean the way all of them are talking there seems to be plenty of work and judging from Cicero's journals it had been years since there was a Listener. So how did they find out who was doing the Black Sacrament? It's not like people advertise it. I figure that Aventun Aritino was being obvious because he was only a kid...

All of this was explained actually.

They've basically become a group of loosely organized assassins for hire. People still preform the Black Sacrament, but the Night Mother doesn't judge whether or not their request is worthy; the DB just takes on the contract regardless of whether it's worthy or not as soon as they hear about the person doing the Black Sacrament.

They've really got no right calling themselves the Dark Brotherhood.. at least not until Astrid is gone.

This. ^^^

Talk to Astrid. And for that matter, didn't you notice the guard mentioning the kid in Windhelm practicing the Black Sacrament? Hell, if the guards know it, everybody does!

people talk, word gets round befor you know it someones dead...

Word by mouth
http://www.uesp.net/wiki/Skyrim:To_the_Brotherhoodessentially complaining that the Black Sacrament that she performed wasn't being conducted.

What was the explanation? It's been a while since I finished the questline and I've only done it once so I haven't memorized everything that was said - there's so much to remember.

I just assumed they got careless and people just 'knew' who to contact. Plus, by wearing the black and red armor/clothing with a black handprint on it and people know who to ask.

They just heard about people doing the Black Sacrament by word of mouth. It works in the same way that everyone in Windhelm knows that Aventus Arentino has preformed the Black Sacrament.

Ah. Yeah, apparently people outside of Windhelm have also heard.

I think everyone but the victim has heard about it, and knows who the mark is. I just can't imagine anyone over the age of 12 being that obvious, though.

OTOH, this is a culture where a serial killer is completely ignored because of a civil war, so I suppose people wouldn't have much problem telling their friends about the cool Black Sacrement they did last night to get rid of their boss.

The Black Sacrament is only a ritual to call The Dark Brotherhood. You only need to mention the target to the Speaker who came after The Black Sacrament. Since civil war is raging everywhere in Skyrim, it's not strange at all if someone want somebody to be killed. So no one really knew who the target was except DB assasins and the one who perform the sacrament (Well, except that blabbermouth Muiri's target).