I think it's safe in the sense that amazon has one of your better return policies should you be dissatisfied with the items condition (not so sure about dissatisfaction with it's performance, if not clearly defective). They used to be great, but I think they've had to tighten up some, probably because too many people taking advantage of them.
It's probably less safe in the sense of being satisfied. I've never bought an AVR from their Warehouse Deals store, but based on all the other products I have bought from WD, I wouldn't trust their product evaluation for anything as sophisticated as an AVR, when they can't seem to even spot obvious surface damage on so many of the used items they sell. Unfortunately, calling for specifics does no good, as their CSRs have no access to additional item specifics. I think, if it appears to work, Amazon just throws it up and assumes the risk, if it doesn't - which they're very good about covering you for most reasons given for wanting a return.
They even let me return a receiver I bought new, 5 months after purchase once. Though, it was their fault for shipping me a supposedly new unit that had the factory seal broken, making the warranty invalid. But, it was a gift, so I didn't expect it when it arrived - being new I'm not sure I would have caught the broken sticker, even if I had opened it myself, assuming the box wasn't also open or obviously resealed.
I'd ask about warranty before buying too. If they normally sell the brand, then they should be authorized, so any open-box return should still be covered, as long as the previous buyer didn't register it.