Read the listing thoroughly
Avoid non-US sellers
Only buy from listings with excellent pictures
Compare listing pictures to verified images
If anything looks "off," even if you don't know why, pass
Only deal with more established sellers
Only deal with those with good return policies
Read feedback
Well, those were some rules I employed. As far as questions to sellers, I've asked if they guarantee authenticity for life, is the coin cleaned, does it look cleaned, how did you get it, how long have you had it, will you take $x for it, maybe some others. I can't really say any answer changed my mind on anything. I always went with my first gut impression, so if I had any concerns at all, real or perceived, I'd pass. Worked great. There will always be another one soon, almost always a better one is coming too. Good luck.