Banking and selling in Neriak is easy once you are 34 and have Call of Bones: Fill 3 or 4 backpacks with red wine and do the
Bottle of Red Wine quest.
The guard faction needs a lot of turn ins to raise to dubious (1 bottle at a time. Using 4 x 1 or even 4 x 4 doesn't work, probably a bug). By the time you are dunious with the guards (doesn't apply to 3rd Gate, guards there can be on guild factions as well) the vendors should consider you as an ally. - Just con Lokar, but make sure that you are safe with the guards as well when you are done.
Call of Bones alone raises faction in Neriak enough to be not killed - with the notable exception of the Necro and SK guild sadly. They are not deceived by the illusion. In Neriak Commons roams a wizard who is on the Necro guild's faction. Once you are down in the Underground you are safe. Be careful or use invis when you are running around above.