Rocket League Items credits the 500 credits included in the from xingwang's blog

First, you’ll need to have purchased at least 500 Rocket League Items credits (the 500 credits included in the starter pack count). If you purchased Rocket League before the free-to-play update, you can ignore this restriction.

You can only trade with players on the same.

 platform as you, but because Rocket League has cross-progression, you can log in to any platform to see your tradeable items. There are some items you can’t trade, including unrevealed Blueprints, items purchased from the shop, and bonus gifts. Similarly, you can’t trade credits for credits or credits for nothing. It has to be credits for an item, and only one player Buy Rocket League Credits can offer credits.