Bob I told you that you send him $? I just told you that you can deal with him yes or no?
You are vouching for the user which IMO make you responsible for any transaction done with this user. You tell the guy he is safe to send money to and if something goes bad you will pay it. That is what vouching IMO should be. Don't open your mouth if you are not ready to pay if something goes wrong.
This is all assuming that you are not the same person, which while I continue reading would be hard to believe.
Either way, if you are not this guy you vouched and owe the $300 or you are the scammer and owe $300.