I really like this one specific bike, which is worth only 15k. But, I don't have a garage. If I purchase the bike online, where will the bike go? I don't have a house, either.
You can't buy a car if you do not own a garage to store it. The cheapest garage is a 2 car garage titled "Unit 124 Popular St." that will cost you $25000.
Just like in the real world, you can't just go leaving your cars all over town!