Not really the easiest way but the most efficient one i found to be working on any level(after reaching level 2) is simply to "abuse" the Auction system and the exchange system, while it does require some game knowledge and brains to do it, simply buying important resources such as crystals, coal, skulls, even plants, dust and chests on exchange for as low price as possible and reselling those on auction for a higher price for people that are in urgent need of it and dont want to wait and try their own luck on exchange will give you a good amount of money, for example buying rag and metal on exchange for a low price before an seasonal event will accour where you need to hand these resources in will increase their worth once you get there, i managed to sell of rag and metal for 4x the price i bought them for during an inflation period to then sell them during christmas where you would need those to get snowflakes as exchange.
as a little warning, some people might think this is a little douchish and i must agree with that to a certain degree, as i am really annoyed by these players that simply overbid things on exchange by maybe 1c and most likely arent in need of these resources in the first place, but where did annoying people ever stand in the way of peoples money? And in the end, that is just how an open market works, demand sets price, buy for low, sell for high.