This exchange is a good example of why this post needs clarification on types of knowledge that should be sought here. "how do you buy a used car" is not a simple procedural question that can have a clear answer in a web posting. It's a VERY large set of options with pretty widely varying constraints and preferences. Not unrelatedly, it has a large number of people making a living in helping people do this.

[edit: I retract this, as I've just seen some useful advice on horribly complicated topics. Please ask (and answer!) whatever you think might help. ]