On this page of the Valheim guide, you will learn where to find Valuables which you can sell to get Coins.
At the time of writing this guide, it is possible to obtain several valuable items in Valheim: Silver Necklaces, Amber, Amber Pearl, and Ruby.
All Valuables can be acquired by exploring underground crypts like the Burial Chambers in the Black Forest or the Sunken Crypts in the Swamps. However, before you go to these places, make sure you are properly armed and have enough food on you, because you'll encounter some really strong enemies there.
The second way to collect valuable items is to search for Buried Treasures. If you don't know how to find the Buried Treasures, we have prepared a separate page on this topic in our guide.