You don't. Gifting is all that is available in GT5. Players have taken this and built a make shift form of trading around it. There is no reliable way to prevent a scam except to not trade.
You can prevent loss of valuables by backing up your data before sending anything. If you're scammed, you can reload the data and the car you sent will still be in your garage. I do this all the time, whether or not there is a threat of scamming. First I back up, then send my car/item, then reload my back up save, then I accept the gifted car/item from the other person I'm trading with. It just makes things easier.
If you are going to use the data backup option, do not accept any cars prior to restoring your old data. You will lose them.