Using assets that other people made is fine. After all, whether you employ them to make the asset, or buy it in store doesn't make any difference. The same applies to free assets. It's only developers who frequent these stores that will recognise the assets - and itch.io tends to be a community of developers (so this might be the wrong place to ask).
Where it becomes a problem is when you buy a demo of a game, and release the game as is. Google Play Store blocked this because they were getting 1000s of listings of the same game. They've now stated that only the person who created the demo may publish it on the Google Play Store. It was this behaviour that gave asset flipping a bad name.