What is a good item to sell to get gold coins? [duplicate]

An easy way to earn gold early game is exploring the jungle. Items in chests will be worth around 1-5 gold. Some may help you, most a sellable.

If you have a Slime Statue or Fish Statue you can use wire and have a 1 second timer to killed a lot of the mobs and get their items. If you can go to the Lihzard Temple and get spiky ball traps, place them down above the Statues and connect them to the 1 second timer. I call it an AFK area. I usually just duplacate my gold though. Hope I helped

There are a variety of things one can do to efficiently make money, depending on how far in the game one is. A guide on money making, conveniently even specifying the amount you want, can be found here: http://terraria.gamepedia.com/Guide:Making_money#Crafting:_Making_1_Gold_Coin

Two examples using extremely common and easy to collect items involve Sand Blocks and Cobwebs. While these items have no sell value on their own, they can be crafted into other items which can be sold for decent amounts of money. Sand is of course in great supply in desert biomes or the oceans, and cobwebs can be found all over the place underground. Sand can be crafted into glass and then into mugs at a furnace, and then mugs can be crafted into ale at a keg. Cobwebs can be crafted into silk at a loom. Then simply sell the resulting items to the Merchant or any other shop NPC. 1000 sand blocks or 350 cobwebs are sufficient to receive 1 gold coin. While collecting sand in the desert, you can also hold on to Cactus to craft Cactus Swords at a workbench, another easy source of money, though in lesser supply.

Alternately, if you've already defeated the Eater of Worlds or the Brain of Cthulhu (depending on if your world has Corruption or Crimson), a single fight against the Eye of Cthulhu should be relatively simple and give you more than enough money, plus a good deal of Demonite/Crimtane Ore which can be sold for even more. The Eye can be summoned at night using a Suspicious Looking Eye, which are sometimes found in chests underground, or can be crafted at a Demon/Crimson Altar with 6 Lens items from Demon Eyes.

Goldfish... It will give one silver each. One of the easiest ways.