When selling to an NPC merchant they will haggle around 10% of the item’s value times the tier of the characters level that is selling the item. If sold the money is deposited in the world bank for the character. Note: NPC merchants keep quasi logs of what they bought from whom, you might get questioned about this later by an official.
When you buy something, it costs the full amount unless you are a member of the guild or the faction that you are buying the item from. If you are a member, you get a 10% discount on the items value times the tier of the characters level that is selling the item.
Black-market items are always at full cost (sometimes more) when you buy. This is mostly stolen goods that no one questions. If you are selling to the black-market, they might pay what a merchant would, but they will keep their mouth shut about who they got it from. If they want it bad enough it might be twice what a merchant would pay. You are expected to pay with and receive gold (not the world bank) when trading with the black market.