The IMHO easiest way to make a bit of extra cash is to cook stuff you already own before selling - it might not be “get super rich super fast” useful, but requires basically no effort. In BotW, a meal or elixir made with 5 items sells for approximately 2.8 times the value of all raw ingredients combined, so you basically get thrice as much cash just by tossing stuff into a pot. I haven’t checked the multiplier in TotK yet, but stuff still gets “more valuable” if you simply spend a couple of seconds at a cooking pot, and the new recipe selection thing makes it faster than having to manually pick items from your inventory every time.

Personally, I also sell Luminous Ore. There are plenty of ore deposits all across Hyrule and since I don’t like the one armor set they’re needed for, I always end up with a surplus. You can also feed them to Dondons and they might poop out more valuable gemstones sometime later, but I don’t like to gamble and rather just sell the L.Ore directly.

Oh and Fire & Ice Wizzrobes are easy prey as well, as they die to a single hit of the opposing element - toss a Fire Fruit at a Blizzrobe and it will go poof immediatly. You can then pick up their “rods”, travel to Tarrey Town, pay the Goron kid 20 rupees to take the thing apart, and you will get a ruby or sapphire each time. Works with electro Wizzrobes as well (Topaz) but they’re not as easily OHKO’ed as their brethren.