If I sold brkge acct stock and put it in my marg acct why do I have to pay taxes on it if it's still in the marg acct? Won't I be taxed again if I withdraw cash from it?

the taxable event is the stock sale.  moving your money from one of your accounts to another is not a taxable event.  thus moving the money from a brokerage account to a margin account (which by the way is a brokerage account) isn't taxable and neither is moving the money from your margin account to say your checking account.