In SA, the previous owners details are displayed on the ticket in the window, and have to be provided upon request.
If you are going to ask the question about trade-in, then give them the option of not answering. Some people are sensitive about it, others don't care. You may even find that someone is happy to tell you, so that you have more bargaining power with the dealer.
As for how often to call, if you have rung once & left a message, leave it for a day or 2, and then try again at a different time. Just remember, don't ring at dinner time. That is the fastest way to get people off side. Over-do the politeness, at least to start with, and you will probably get the right response.
