Look closely at the pictures, and don't be afraid to ask for more pictures and ask questions. Ask about Social Security numbers and/or driver's license numbers etched into the gun in different places. This is one that is so commonplace and it absolutely kills me. Why in the world people do it I have no idea; the gun had a federally mandated serial number, but they do do it, so ask.
People's ideas of what good and excellent are vary considerably. I got a "very good condition" Model 1100 that was truly 99.9% plus. I got another gun that was listed as 95% and I can find nothing wrong with it beforehand, or later with it in my hands and disassembled no less.