Jack, the scammers somehow hack into legit sellers eBay accounts. This allows them to post on the sellers accounts for a short period of time until it gets yanked.
I am only guessing here, but I suspect they change the sellers password and use the account, locking the real sellers out of thier accounts. Once again, I haven't a clue HOW they do this.
Ebay will send an email to the sellers registered address infroming them that thier password and email info has changed. Since THIS is also a common scam to get sellers financial info, I bet most sellers just delete the email and and find out later it's true when they try to use thier accounts. By that time, the scammer is long gone.