The only way to earn in-game money is through selling items. You can't buy in-game money with SparkCash.
In fact, you can play the entire game from start to finish without ever spending a dime.
SparkCash allows you to buy optional items from the Outspark Store such as consumables - potions, health boosters, xp boosters - visual upgrades for you house and mount, and outfits. Of course, these items make the game easier, but aren't really necessary.
If you want to attend the game's daily in-game events (which you can check out at the event calendar
here), however, you often need a promotional item. There are free events too.
Once you get some levels, be sure to attend some of the free events. They're a ton of fun, and if you like what you see, you should check out the item-based events too. They're my favorite part of the game.
