Psyonix declared that doing trades outside of the Rocket League game puts players at great risk. They do not encourage selling items for real money. Now, trading can also be done in the game between actual players, and this is a much safer process. But if the players trade outside-game, there's not a lot that Psyonix can do about it.
At the same time, many trustworthy platforms let you do Rocket-League trading outside the game, and they are perfectly legal and reliable. But before you start trading, it's always best to check the laws about trading in your area. Not only that, but people who are doing these trades are unlikely to be discovered.