Ok, the choices I found that somewhat meet these requirements were:
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Nether Vortex, stack 200, buy 70, sell 2g, G'eras Shattrath 50.8 42.4, Crafting Reagent .
Primal Nether, stack 200, buy 40, sell 1g60, G'eras Shattrath 50.8 42.4, Crafting Reagent.
Hardened Adamantite Tube, stack 200, buy 12, sell 3g, Dealer Najeeb Netherstorm 44.8 36.6, Crafting Reagent.
Elixir of Tongues, stack 200, buy 23g96, sell 7g49, Fizzi Liverzapper Dalaran(new)UB 76.2 82.4, Other.
Ironspine Protocol, stack 200, buy 9g85, sell 2g46, Clockwerk Storm Peaks 33.8 58.0, Other.
Primordial Saronite, stack 200, buy 135g, sell 7g, Arcanist Uovril Dalaran(old)UB 38.8 53.2, Other.
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Even though its not a reagent, I'm gonna go with Elixir of Tongues; because, Fizzi has an unlimited supply, the sell/buy ratio aint bad, and he's just moments away from the spot the Dalaran hearthstone drops us.
The portal stone drops us at the base of the walkup to Krassus Landing (the flight master's area). From there, take one step into the city and look to the left to see the ramp down to the sewers. Once down there where the elven guards are take the first usable pipe on the left into a room lined with wooden ladders and platforms. Go up the far corner on the left then through to the tunnel behind where Fizzi has a laboratory. Just buy all the Elixir you can to keep well away from hitting gold max. Keep one stack in your bags in case you ever need quick cash, and drop full stacks in the bank to create a retirement fund.
