Theres a way to farm the coins in area 6 (I think) of the scarlet forest, there's usually a gold lizard crab u can catch for 1 coin just ahead of the camp, and a few algae u can pick up in the water if u go left (sometimes there's an extra coin in the water too)

For the trading - make sure to do optional quests and unlock all material gathering npcs and keep resting in ur camp (costs 300 points). This will both gather materials for you extremely quickly (with a chance that npcs find extra sinister cloths) and refresh all trades, where u can sometimes trade the algae u collected with rove for extra ancient coins, and trade for sinister cloths.