To become a Goblin Killer, be a Mage, Priest or Scout and kill alot of Grutooths.
To become a Goblin Slayer, be a Warrior. You have a low chance of becoming one.
Do wholesaler's tasks (Tradeskill quests). Tier 1 quests give 3 silver and Tier 2 give 12 silver. If you do not want to spend time harvesting for the quests, buy off the broker.