that happens to me a lot, just explain to them what happened and if they sent first, just try and return there car, I've learned that late at night when most people are sleeping is a good time to trade because the server isn't as busy
I had four cars get lost and felt terrible about it. I had doubles of 3 of them so I have sent then my extras in the meantime. The fourth person was a little salty because it was a 908 but I will be sending his 2 cars back as soon as I can. I honestly feel like the responsibility should ne shared because neither person knew it would get lost. I personally didn't want the other person being like I lied to them so I went out of my way to make them happy.
If a car is lost during a trade the best thing to do would be to gift back the other persons car or arrange forr another vehicle/s to be traded in its place.