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]
Mens rea means you intended to do the criminal thing that you did, not necessarily that you knew it was criminal. Homicide is homicide because you intended to kill someone, even if you didn't know that was a crime; if you did not intend to kill someone, you lack the mens rea but may still be guilty of negligent homicide or some related crime.
Certain crimes do require knowledge of the law, like tax evasion: the tax code is assumed to be so complicated that to be guilty of criminal tax evasion it must be proven that you knew you were evading taxes. Wikipedia has a summary: https://en.wikipedia.org/wiki/Mens_rea#Ignorance_of_the_law_...
if you did not intend to kill someone, you lack the mens rea but may still be guilty of negligent homicide
The standard varies by jurisdiction, but it's quite common for "intended to cause non-lethal injury but accidentally killed the victim" to be treated as murder; this arises for example in the "eggshell skull rule".
Some states also have what is referred to as "constructive murder" whereby you can be convicted of murder if you commit a serious crime and someone dies, regardless of whether the possibility of a death was reasonably forseeable; in Canada this has been ruled unconstitutional on the grounds of lacking mens rea but in the USA courts generally accept that having the intent to commit a serious (but non-lethal) crime meets the requirement.
“For example, 20-year-old Florida resident Ryan Holle was convicted of first-degree murder for lending his car to a friend, who used the car in a burglary during which a murder was committed.”
The eggshell skull rule is from torts, not criminal law and it would be unconstitutional to apply it to criminal law. You're thinking of constructive murder, where the intent to commit a serious felony everyone to the intent to cause all foreseeable harms that the attempt could engender.
Different countries, different constitutions. In Canada, constructive murder was ruled unconstitutional; but if you intend to shoot someone in the leg (say, to stop them from pursuing you) the lack of intent to kill would not save you from a murder charge.
In addition to the comments about mens rea being an intention to do an act, not knowledge of the law, sometimes your knowledge (or lack thereof) is part of assessing your conduct. For instance, the Englad and Wales Theft Act:
> A person’s appropriation of property belonging to another is not to be regarded as dishonest if he appropriates the property in the belief that he has in law the right to deprive the other of it
An honest mistake as to the law would not be considered theft.
https://en.wikipedia.org/wiki/Mens_rea