I don't think it's unreasonable to give it back if it stinks. That's disgusting.
I'd shut it off, remove the screen protector, and clean the shit out of it. Reset it to its factory settings- this is not difficult. Follow the on-screen instructions and get rid of the old account that's on there.
I never buy used portable gaming systems unless there's no choice- years ago I needed a PSP on the right firmware to use CFW, so I went for a used system only because I had to...and that's it. With a console you can give it a good cleaning and replace the controllers with new ones, but with a portable you're stuck with whatever kind of abuse might have been done to the buttons/controls.