Regarding Yamaha's
warranty coverage, it's located on their website. To quote part of it:
How can I be sure that I will get the Yamaha Manufacturer's Warranty?
Yamaha sells its Home Audio products only through a network of high quality, specially authorized dealers. The Yamaha Limited Warranty applies only to products purchased from our authorized dealers
There are various reasons why someplace will have it cheaper. One reason could be that they are refurbs, but if the place says that they are new (and you know that you can trust them), then they probably are. Receivers don't cost the stores MSRP to buy them, so places that just have them drop-shipped to you from a warehouse have very little overhead and can still make a profit while charging you less.