In the Rocket League Terms of Use[www.psyonix.com], under section 5, paragraph 1, it is explicitly stated that selling items without the prior consent from Psyonix would be a direct violation of the agreement.

All intellectual property rights to the Proprietary Material, including patent, copyright, trademark and trade secret rights, are owned or licensed by Psyonix. You agree not to copy, download, reproduce, republish, upload, post, transmit, perform, display, distribute or sell, or in any other way exploit the Proprietary Material, or to participate with or to encourage others to engage in such acts, without the prior written consent of Psyonix.

With this being said; any and all threads and comments that lead to real money trading will be either closed or deleted. Those who have instigated such situations are likely to be banned from the forums.