You can sell your yellow gold for cash. There are a few quests that require certain kinds of gold chunks, but for the majority of the time, it is completely safe to sell any and all of your regular YELLOW gold.
NOW - the big exception to this is WHITE gold and Trigold nuggets. These are rare(ish) and need to be used for some tasks. Keep a safety net of them around at all times.
The other way to get yellow gold fast is to turn in wooden sets, they will net you one 100 ounce nugget and one nesting box. You can then sell the yellow nugget as above.
Technique 1 - Being Thorough
Check your station for exposed hides and open them. Check your station for golden animals hiding in the brush. Check your collections and exchange them. Several collections net coins straight, others give you gold that you can exchange for coins.
Technique 2 - Cucumbers and Large Hotbeds
Tip is best for lower levels: Once you unlock the large hotbeds, plant Cucumbers and sell them for coins and slowly buy more and more large hotbeds. Cucumbers purchase for 255 coins and sell for 663, netting you 408 coins. Also works well to net you the coins for Greenhouses necessary to plant higher level seeds.
The reason why this is best for a lower level player? If you have opened the Forgotten Trail land, you can use those cucumbers to make water in the well, making them a valuable commodity.
Technique 3 - Fruit Trees
if your level is high enough, you have some kind of source of Water, and 50-70 thousand coins in your savings, start planting fruit trees. It costs 50,000 coins to buy a Pear Tree and 70,000 coins for a Cherry Tree. they have a grow time of 12 hours and produce 1 fruit each. Pears sell for 3,000 each and Cherries sell for 5,000 each. It uses 2 water per tree when watering them so you go through water very quickly (so have a source of water like a bunch saved from your friends or the water well). Trees can also net you worms, getting you valuable trading currency or items to raise your birds.
Side Technique: Saving Money with Fruit = More Profits
Related to Technique 3, you can exchange 5 Pears from your Pear trees and 5 milk at Wind's Song for a tribal chicken instead of spending 50,000 coins to purchase it, making an extra 70,000 profit on eggs.
Technique 4 - Eggs
Selling eggs is one of the most common way of making money in this game. Turkey egg containers sell for 20,000 coins each, making them an easy way to make coins. Goose egg containers sell for 10,000 coins each. These are an easy way for even low level players to make coins as they are accessible fairly quickly. Various guides exist explaining what is the most effective.
Ostrich Egg Baskets are typically the way to go for quick and easy cash fundraising. Each Egg Basket sells for 80,000 coins and can be made in Polar Side.
Note: there is a great guide to poultry profits on the game guide here.
Technique 5 - Selling Antiques
Most of the time, it doesn't pay to sell your statues or other antiques. Except for the Goddess and the Hand, all of the statues and antiques are needed to make other items in the game. So hold onto them! Your Goddesses can be sold for 17,000 each and the Hands can be sold for 16,000 each. Keep about ten of each on hand just in case they are used in the game in the future, but sell off the rest.
Technique 6 - Rabbits
Rabbits. Rabbits sell for 5,000 coins each. Highly recommend reading the game guide linked about them. They are not uncontrollable, and can be a nice source of ongoing revenue if you have the time. Their downside is your initial investment is 100,000 coins, so you have got to have quite a savings going before you can purchase them.
Technique 7 - Visiting Friends
If you have not already used up your daily shovels and/or lucky shovels, you can often find some extra gold at your friends' stations without using any energy up. Using your shovels on pyrite in particular can net more gold. Selling the gold then nets you some coins (varying depending on the nugget awarded). And of course if you hit a gold vein, you'll get a lot of gold to sell!
Technique 8 - Indigo Sets
Exchange Indigo sets (which can be gained from Peafowl nests and statues) for large amounts of coins