If it’s a private seller, don’t leave until they’ve transferred the car to you on the app, it takes only a couple of minutes. Before you get there set up a Tesla account, download the app and log in. There are a few guides around including at least 1 YouTube video from memory, but it essentially requires them to go to their products section in the app and remove (transfer) to you, they enter your name and email address, press go.. they get emailed a 6 digit code and within a few minutes the car will be in your account.