If the spring is real tight when all the way loaded, leave one out. If the mag rattles with one shy load it all the way. Never overload the mags. You can find glock mags that will overload, but don't.
Your best bet would be to set and dry fire the crap out of it when your watching tv. you Will save a hundred bucks and strengthen your trigger finger as well as build mucle memory. And in the end have a nice action job to boot.
You do not have to transfer it thru a dealer. You file teh transfer paperwork thru ATF and you maintian possession of the weapon until the new paperwork is issued. ATF should send the new paperwork to you, and then you give it to the new owner with the weapon.