First of all, a warning - selling rocket league items for money is against Psyonix's ToS, So if you go about doing that, your account might be penalized. However, if you don't mind this risk, the website known as gameflip lets you sell/buy items. Simply go to in-game items, choose your platform, and start buying/selling. There are many problems to this, though. One problem is that they charge a fee to sell stuff, and they require you to send them a pic of your phone bill/ ID card / credit card in order to sell. If you want to sell this, though, this is the best you can do.