Go to your garage - select a car - underneath it you get a row of things you can do... Make it a Favourite - PLace in the online area - get into the car (Change car) and alter the settings.
IF you put it online it means you can't use it yourself as it is "frozen" and anyone who is your FRiend and has GT5 can borrow your car - exactly as you left it.... When you want the car back it will be just as you put it there. - Useful if you want to borrow a Chapparal 2J or some obscure car for some race (The Ferrari F1 series, maybe the Formula GT event - handy if you don't have loads of cash -)
I'll paint it some obscure colour - you can always paint over it.
Basically trading is blind gifting on an honour system. Many "duplicate" cars by saving their game on a flash usb lego brick. then send the cars/paints/horns... to someone. then they quit the game and then copy over their PS3 save with the saved one form the usb brick...
Whatever you can only trade one car/ticket per day - and that day starts and ends at 1am at night.
Keep having fun. alot of funny folk - but mostly helpful