I guess its more or less what you feel comfortable with. I bought my hk230 from harman audio on ebay in may 2004 and never had a problem with it. It appeared to be brand new.
Make sure you buy from an authorized reseller so your warranty remains intact, and you don't get left empty handed. What are you looking for btw? Maybe we could help you find a receiver for a good price.

*Michael*
AV123 Refugee - X-LS Encore, X-Voce, X-Omnis, Elt-Dpa's
Denon AVR-591
Magnavox NB500MGX BDP