A big issue indie developers face is copyright. The creator of Flappy Bird was supposedly sued because he was making 50,000 a day on ‘ripped textures’.
In all honesty - it only matters if you intend to go commercial with your work (physically selling it). If you DO - and the content you’ve used hasn’t explicitly been labelled as ‘Free to use’ - I’d stay away from the materials of others. But you can always contact people and ask! Sometimes they only want a mention in the credits.