Can I use those items in games that I wanna publish online (and hopefully one day sell for money)?
If you purchased them online in the browser you need to go through the launcher market place and add them to your vault.
Once you have added marketplace items they will be in the vault at the bottom of the Launcher->Library tab. You have the option to install them to your pc from there. You can then add them to your project.
Yes you are allowed to use things purchased of the marketplace commercially.(Not sure on infinity blades case though…)