There is a new trading exploit going around (Currently unconfirmed by Psyonix). It may be best to avoid trading until further notice. We will do our best to keep this post updated with any news regarding this exploit.

Copy/paste from r/RocketLeagueExchange:

scammer does something which glitches the trade window to keep a certain item showing when it actually is something else.

so far the common tell is that when you are on the 2nd trade window, if you ready up and the timer starts counting down when the scammer is NOT readied up. when you back out to the first trade window you will see the actual item that the scammer had in the trade.

NOTE: this does NOT mean that you are necessarily safe if the scammer readies up first and it looks normal. there doesn't seem to be a full blown way to tell what is actually in the trade window.

Here is a clip of the exploit in action.