If anyone was in the Rocket League Alpha, then you are likely sitting on a goldmine. I was in it and it gave this ugly as

gold hat as a bonus. Well now that stuff is tradable, and the thing is selling for big amounts. I sold one last night for $200. The one thing that's a pain is that its in game trading so you have to be really careful of potential scams. I found a buyer who was willing to go first and had feedback (the fact I have high feedback on multiple places helped), so obviously aim for that if you're diving in this pool. I had no idea til I looked here
https://www.reddit.com/r/RocketLeagueExchange/ and saw what things are going for. I got my first crate drop and was looking around to see if the dev is ever going to allow marketability/tradability and discovered the Exchange then. Tradability is live now inside the game itself, but no word on marketability so every thing is user to user at the moment.
If you were in the beta (I wasnt) there is a slightly less valuable "golden nugget" that was given as the bonus for being in it and its not selling for as much as the hat, but its still going to get you a nice chunk of change. I saw one go in a thread less than 24 hours ago for 50 keys.
If you've opened any crates (I havent) be sure to take a look and see what your drops from it may be worth. Things like decals and especially cars seem to fetch a lot at the moment.