Just like going to Vegas and playin the slots..
Pull the handle and hope for the best..
Buying refurbs (B-stock) one can save $ but has its risks..
If there is a technical issue, who pays the freight?
Make double sure all of the accessories are included such as AC cord, primary remote control, EQ microphone.
Also verify its warranty (parts, labor, shipping costs), noting that most issues show up in the 1st 2 months.
I've bought 4 refurbed receivers from ac4l.com over the years without any problems. Great value compared to new. I've also had very good luck with used receivers. But everything comes with a risk. New, used, refurbished. These things are getting real complex. It's all a gamble.
I don't see the Onkyo 818 anywhere for $500. That would be a really great deal.