NOTE, this cannot be done early in the game. Players with late pre hardmode gear or
better are strong enough to be able to farm money with this technique!
If you are low on money and miss an item you really want from the Traveling Merchant, then you wont miss anything ever again after reading this post. Demonite bars and Crimtane sell for a very good price when put in large stacks. 1 stack of 99 bars of Demonite sell for 31 gold, and 1 stack of Crimtane bars sell for 39 gold. A very easy way to farm these is to make a world and to into the evil biome then summon the EOW/BOC by breaking the shadow orbs/pulsating hearts. If you are playing on any platform that has received the 1.3.2 update it is recommended to pick the crimson as your evil biome because Crimtane bars sell for more than Demonite bars, but you can only do this post WOF. Making stacks of bars is more fast in Expert Mode but bosses are more stronger in this mode. I hope this has been helpful to you, but i know many of you will say that this is cheating and i agree.