Is it permissible to open, sell, or buy free loot boxes in the game Rocket League? Is it permissible to buy and sell the items within them? And is it permissible to sell "credits" for real money?

Selling points for permissible games is allowed if they have a known benefit. As for buying and selling boxes and their contents, it is a condition for its permissibility that the content of the box be known, in order to avoid gharar (excessive uncertainty). If the box is free, there is no objection to obtaining it, even if its contents are unknown. It is better to avoid doubtful games, due to the Prophet's (peace be upon him) saying: "Indeed, the lawful is clear, and the unlawful is clear, and between them are doubtful matters that many people do not know. So, whoever avoids the doubtful matters, he has sought to clear himself in regard to his religion and his honor. And whoever falls into the doubtful matters, he falls into the unlawful."