I'll list here all places where you can SELL emeralds.
I'll write down how much STACK of emeralds is costing.
Port Ostia:
Redstone Guy: 1 Compass, 6 Cobble is 1 Emerald; Compass is 4 iron and 1 redstone, and its quite expensive.
To get stack of emeralds you'll have to mine 4 stacks of Iron, 1 of Redstone and 6 stacks of Cobblestone.
Misc Items Guy: 10 Sugar, 12 Wheat is 1 Emerald; You can use 1 bonemeal to obtain 1 wheat.
To get stack of emeralds you'll have to farm 10 stacks of Sugar and 12 stacks of Wheat (you can use 4 stacks of bones).
Catharge (Spawn):
Weapons Dealer: 14 Flint, 4 Cobble is 1 Emerald; Use Fortune III shovel to mine gravel.
To get stack of emeralds you'll have to excavate 14 stacks of Flint and 4 stacks of Cobblestone. Cobble is dirt-cheap.
Weapons Dealer: 15 Sticks, 5 Feathers is 1 Emerald; Do you have huge feather production capability? PERFECT!!
To get stack of emeralds you'll have to execute 3.75 stacks of Wood and 5 stacks of Feathers. Sticks are dirt-cheap.
Minecart Shop: 14 Gold Ingots, 6 Sticks is 1 Emerald; HELL NO, unless you want to get rid of gold.
To get stack of emeralds you'll have to drill 14 stacks of Gold Ore and 1.5 stacks of Wood. 14 stacks of gold ingots... >.>
Tools Seller: 14 Coal, 3 Cobblestone is 1 Emerald; Use Fortune III pickaxe!
To get stack of emeralds you'll have to dig 14 stacks of Coal and 3 stacks of Cobblestone. Coal is almost dirt-cheap.
Food Stall: 20 Wheat, 13 Sugar is 1 Emerald; LOL OSTIA IS CHEAPER!
To get stack of emeralds you'll have to farm 13 stacks of Sugar and 20 stacks of Wheat.
Inn Keeper: 2 Glass Bottles, 3 Cloth is 1 Emerald; Do you have huge wool production capability? EXCELLENT!
To get stack of emeralds you'll have to get 6 stacks of Glass and 3 stacks of Wool. Glass is dirt-cheap.
Armour Guy: 4 Iron Ingots, 3 Leather is 1 Emerald; Do you have huge leather production capability? Good!
To get stack of emeralds you'll have to obtain 4 stacks of Iron Ingots and 3 stacks of Leather. Hope you like to mine.
Roxbourg:
Potion Ingredients Guy: 4 Ghast Tears, 21 Spider Eye is 1 Emerald; This guy is high on something.
To get stack of emeralds you'll have to kill 4 stacks of Ghasts and 21 stacks of spiders. >.>
Miner: 16 Gold Ingots is 1 Emerald; If you want to get rid of gold, but Catharge is cheaper.
To get stack of emeralds you'll have to mine 16 stacks of gold ore. Again, deal for far future.