Crowns is a soulbound currency, which means you can't sell it directly, but it is an important reagent for upgrading Heroes (tradeable NFTs), which in return results in you receiving better chests, which in return contains better loot.

There is another soulbound currency within the game, Ethershards.

The difference between Crowns and Ethershards is that Crowns are earned roughly the same across all rarities, especially in regards to Quests & Leaderboards, while Ethershards are influenced by your skin in the game - i.e. collection rarity. The latter is critical since Ethershards are required to un-soulbound items and forge tradeable items. Crowns are earnable in free-2-play, Ethershards are not.

Forge: https://ethgardlegends.com/forge
Items: @ethgardlegends/ethgard-legends-is-now-live-on-opensea