This is not GTA, dude. I don't think this is the point of GT5 at all. To try cars before having them there should be test drive, I think. Or you can get one from a friend who have a specific car you want.
You can share and borrow cars online. Beyond that there's gifting. GT is a game of driving realism. It is not The Real Car Buying Simulator, despite its many models. There's no Financing-spec, for instance.
Or you can have an option where you can send a message to another player and trade cars(While in a lobby), and you can set a time limit for how long you have the players car, or when one of them leaves the lobby, you return to your car.
Or very simply give an option for the host player to allow open access to his garage, so that when any other player in the room opens their garage icon, a folder with the hosts garage is shown.