Buying a Samsung Galaxy S8 so close to the S9 unveiling may not be smart, but a great deal is a great deal. The G950U model US, unlocked is available for $476 on eBay. This is the Snapdragon 835 model and should work on all carriers (the software will change based on what SIM you put in the phone).
These are former display units or excess inventory getting cleared out before the new one comes out. Only Midnight Black units are available and they come with a wall charger, but the other accessories may be missing (you dont get the retail box).
The same seller also has manufacturer refurbished units of Galaxy S8+ (G955U) for $490 and Galaxy Note8 (N950U) for $610.