So at the wayshrine in front of the DB headquarters on the Gold Coast are two Orcs. One sells to lots of dubious characters and will refuse to sell to you if you have a high bounty. The other is Grazda
Spoiler
Later you can find her in the DB hideout commenting on how good it is that the family has grown. /spoiler]
What's the deal with these two Orcs? Are they family or do they work together? Why does Grazda hide out in plain site next to a merchant that refuses to sell to a person with a high bounty?
Well the high bounty is really any shopkeep, likewise if you're a late stage vampire, they also won't sell to you unless you use the mesmerize skill on them.
The Dark Brotherhood thing is that unlike us, the world doesn't know who is a member and who isn't, and the members could always be found in the world, just like in Oblivion. They're semi-regular (as regular as you can be as a murderer) people, and outside of their activity, they try to live normal lives to fit in with the populace.
PC - NA
Spoiler
CP 1125
Veric Blackwood - Breton Magsorc DC
Xhiak-Qua'cthurus - Argonian Frost Warden EP
Kujata-qa - Khajiit Magplar AD
Suunleth-dar - Khajiit Stamblade AD
Teldryn Antharys - Dunmer Flame DK EP
Strikes-With-Venom - Argonian Poison DK EP
Rur'san-ra - Khajiit WW Stamsorc AD
Ilianos Solinar - Altmer Stamplar AD
Iscah Silver-Heart - Reachman Magden DC