Let’s say I bought a $40 asset in the marketplace, could I use that asset to make a game, then sell it on steam, or wherever?
You can use them in your game and sell your game with no problems.
Check outs Usage Rights here: Marketplace Support
You cannot just take the assets you bought for $40 and sell them as-is for your own price though.
I’m not. I’m using their assets, but also adding my own original stuff as well. thank you.