You could always take the gun to an FFL to do the transfer. They might charge you a few bucks but then the transaction would be legit. I actually did this a few months ago through Bass Pro, but that was mostly because I was selling across state lines. Just dropped the weapon off at their store and they charged the buyer $30 or so when he picked it up. Got a full receipt for the trade-in and everything.
PM me with details on that Glock 19 (price, round count, magazines, sights, etc.)