One of my Friends recently gave me some skins on Rocket league, but I don't play that often.
Is there a possible way to sell all Rocket league Items?
There is no officially supported way to sell Rocket League in-game items. However, Rocket League does have an in-game trading market that allows you to trade items with other players, although not for real money.
Note that for most games, any transactions outside the official ingame trading market are not officially supported by the devs and you may even risk getting your account banned when doing such transactions. This might not be the case for Rocket League because the game devs participate in some of the markets.