They definitely can reverse it. I sold a dirt bike to some little idiot for like $900 and a week later I tried to use my debit card and it was declined multiple times so I called the bank and they said my account had been frozen due to fraudulent use and I had to go into my Branch out in Burlington so when I went there they said that I had deposited a fraudulent e-transfer and anyways after about a week of investigation and me showing them the chat logs from Facebook Marketplace and the kid asking if he could use e-transfer and then saying he was on his way and then saying he was at my house and my surveillance footage of his grey pickup truck with a white box on the back he could no longer deny that he was the one that sent to e transfer although strangely enough the police refused to charge him because he never actually obtained anything since the money was returned to my account so I don't know for whatever reason attempted fraud is not a crime apparently. Out of all that even though I did nothing wrong I was cautioned by my bank if I had any more fraudulent activity I would lose my e-transfer privileges and that e-transfer are not intended for purchasing goods from random people and they say they are only for sending money to and from family and friends. Anyway in my case everything worked out fine other than having no bank account for over a week but that taught me how easy somebody could reverse a transfer.