But only certain ones I think... ones you can't buy from car websites in-game. For instance, I really wanted me the Ocelot F260. I didn't see it available for purchase on any of the websites, so I kept the one Simeon wanted me to get for him. It costs something like $12,000 to make it yours ($2k for a tracker, $10k for insurance), then you can do what you want to it. All I noticed is that it can't be put in my garage... that's kind of stupid, but the car *is* essentially mine, since the mechanic will bring it to me if I call and ask.
Sorry if this is common knowledge. I thought it was pretty cool and useful.