I recon this is a really good price for Galaxy S8 Plus. It's a grey import but QD has been quite reliable in the past and has a good reputation. The gold one is the same price and the Black one comes to $815.
PS: I think it's the first time S8 Plus has been available for sub-800. I love mine, it's a great phone and Samsung seems to have reduced the crap they load on in them.