Etsy claims to be rapidly returning the taken funds and paying any penalties incurred by their customers. If they didn't, it could be taken as a signal that they intended to keep it, satisfying the requirement of criminal intent.
If, through an accident, you acquire the property of another, and don't give it back, that can be theft. This is called "theft by conversion."[1]
If, through an accident, you acquire the property of another, and don't give it back, that can be theft. This is called "theft by conversion."[1]
[1] https://www.newyorklitigator.com/conversion.html