For refurbished receivers
Check out the Harman Audio Store on Ebay. Also, ecost and ubid frequently offer good deals on refurbished Onkyo and Denon receivers. The HK and Onkyo receivers come with a good warranty while most of the Denon's give you 90 days.
I would recommend buying a 7.1 receiver and wiring for 7.1 if the room isn't already finished. To save money, you could start with 5.1 speakers and add the rear surrounds later. Also, you could use the extra amp channels on the receiver to power speakers in another room. Most 5.1 and 6.1 receivers these days are low end models that wont have enough power for your room, IMO.