Sealed Vial of Poison starts a quest. It is a really simple quest and pays 13g23s.
You will need to right click on the Sealed Vial of Poison and accept the quest. Then all you have to do is go down into the Underbelly to the Black Market. The person you want to see in the Black Market is Alchemist Cinesra (63,11). He is located straight ahead at the end of the dock when you come in, you can't miss him. If you accepted the quest, as you should have by this point, he will have a big yellow ? over his head. You hand the Sealed Vial of Poison over to him and he gives you 13g23s, quest complete.
This literally only takes about half a minute to ride from Marcia Chase (53,65), by the fountain in Dalaran to the Black Market and talk to Alchemist Cinesra and collect your 13g23s. This is one of the extras they throw in to make this daily more interesting, like the New Age Painting signed by Marcia Chase which sells to a vendor for 25 gold and the Stormjewels that go for 350-750 gold each on the auction house.