Black Vitriol

About 22s 80c on Beta · Horde (6 hours ago), 39s 99c on Beta · Horde (23 hours ago), and 1g 55s on Beta · Alliance (1 day ago), from player scans. A vendor pays 10s 0c, so the auction house pays 2.2× more.

3 recipes: Soul Vessel, Philosopher's Stone, and Alchemists' Stone.

No. It is in Classic Era too, with the same item level, vendor price and stack size.

Sell it on the auction house. After the 5% cut a stack of 20 brings about 4g 33s there, against 2g 0s at a vendor.