One of the best ways to get rich in 'Rocket League' is by buying and selling Ncvr (Non-Crate Very Rares). Non-Crate Very Rares are acquired by playing games or trading up uncommons and Ncr (Non-Crate Rares).
To get rich you need to buy these items and then trade them in and when you get a Ncvr you can trade it for 100 credits.
You should buy 4 or 5 uncommons for 10 credits and 1 non crate rare for 10 Credits or 2 non crate rare for 30 credits.
Keep in mind that a rare is not a non crate rare.
A non crate rare will say it is not from a crate but be careful because some are fakes.
To see the full list of 'Non crate items click the button below'.
One of the best and possibly the best method to getting credits is buying RLCS Items.
Buying RLCS Items as soon as they come out will give you a massive profit if you keep it for a while.
Take Unpainted Apexes for an example They started at 200 Credits and they went up 800 Credits in a few months.
Here is what happened.