I don't really feel all that comfortable using ripped assets personally (including free ones), unless it's for a fangame / clone of an existing IP. Sound effects are somewhat a special case for practicality reasons, but artwork it just feels like I'm being lazy.
I'd rather use poor quality placeholders, and if I ever get a project nearing the point I could consider actually selling it, I'd look into hirig an artist at that point. Or perhaps at that point make the effort to make good stuff.
That's just me though, and I know a lot of devs couldn't care less about this point as long as it fits.