Questions about the Ford Probe II Purchase

Hello, I would like to ask a few questions about car purchases in general and in particular with sample II. I needed to get a sample II before me at the dealer. I would like to have my old cart (Golf II Built. 89) paid at the same dealer because I assume that it will bring me more than if I just sell it like that (is that so?). What is the exact procedure if I want to go (to put it quite salop) with my old cart and with the new one on the same day away again? What do I have to pay attention to when selling my old or buying my new one (or is it all about the same contract)? Do I get all this regulated in one day at all?Another catch that makes the matter more difficult is that my golf has been admitted to my mother for insurance reasons and this should be the same with the sample (what else do I need to pay attention to when buying here)?Where do I need written authorizations from my mother?