I am making a game. It is free-to-play. Is it legal to take the top-hat from the Emoji One (v1) icon set licensed under CC BY-SA 4.0 and sell it as an in-game cosmetic items?
I presume you want to sell (licenses to) software that uses the icons, not the icons themselves. As long as you don't modify them, and tell your users where to get them (and possibly change them for others) and state clearly under what conditions they are being used I believe you are in the clear.