Effort. It's all about Effort.
More Effort = More Results/Money
Restock - Very good way. Depending on items you get from it. Some items can earn you a few hundreds on a purchase and some can roll in thousands and even millions. [Esp Some Medicine] Imagine you restock a Battle Duck at the shop...
Stocks - I do it often. Everyday, you invest almost 15k. However, at end of the month, if the stock goes up by A BIT. You reap in almost a 100% - 300% Profit. Imagine if you bought 3000 stocks. 45k In. 300% Back. 135k in you pocket.
Luck - Imagine if you walk around and get a free Royal Paintbrush? Free Money. Imagine if you got lucky with a Snow Faerie Quest or won at the Tombola. Money too. Random Events reap good benefits too.
Games - I can earn 20k easily daily. Usually i am lazy and get like 10k. Anyone can do it
Shop - Resell items is a good way to earn money. Buy cheap sell expensive.
Clearing Stuff - The person you claim may have cleared his SDB full of retired items. That would bring about A LOT of money.
It's quite easy you see.
Games - 12k
Stock - 150k
Shop - 3k [Around There]
Restock - 500k - 2m [Maybe More Or Less]
Luck - 50k - 250k [Depends On Your Luck]
Feel like got something missing but ah well.
There you go 3m already in a month. It ain that hard