Easiest gold for me is doing trade runs. Get that donkey, in fact doing the quest for the donkey will hand you alot of gold right there.
You don't need to be high level at all to do some of the profitable trade runs. There is a few routes that is quite profitable that is 100% safe , you just need to prioritize what you plant on your farm. I got a farm cart (can do 3 trade packs at the same time) and depending on my route, that is 15g-25g within 30-40 minutes , only thing stopping me from making even more is running out of LP to make the trade packs

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Not sure which continent you are, but i'm on East , so routes like Silent Forest -> Villanelle (climb on airship) -> Falcorth = 8.6 gold for very little effort. Just for kicks i like to sail around the coast from Solis -> Villanelle (climb on airship) -> Falcorth for about 7g .
As for playing the AH, this is where things are different on each server. You will have to see what materials are selling well. On my server, stone and wood prices have dropped like crazy (like 18g for 300 stone) where worker compensation pots as gone up to like 18g . This tells me raw building resources are flooding the market, but there is not enough labor points to process all of that (i.e. turning Raw Stone -> Stone Bricks ).
So if you got the Labor , check what kinds of profits you can make from converting raw materials and compare that with the worker labor pots (that gives 1000 LP) .
Then there are the popular trade runs, check what the ingredients are for the tradepacks on those routes. For example the game has this thing where if too many players are running the same trade route with the same tradepacks the profits drops. So they are forced to run different routes OR make different tradepacks on the same route. So silent forest, which is popular , everyone is making the easy tradepack that uses processed fruit , but if you time it wrong your profit goes from 8.6g to as low as 7g. The alternative tradepack is made from promenades ...so there might be a market for that specific ingredient.
Also , things like Fertilizer/Eco-Fuel (now that everyone got ships and farm carts, they need fuel to go faster), is also selling for quite high prices.
So you need to do some research and see what state your server is currently in. There is even alot of money to be made from flipping properties, although you need an initial cash boost. Pretty much like real life, you buy property for cash, re-sell for a higher price.