The Galaxy S8 and S8+ have been out for a few months, so the sales are really starting to make a dent in the price tag. Today, the dual SIM variants are available on eBay for the lowest price we've see yet. The S8 is $640 and the S8+ is $728. Those are both more than $100 cheaper than the standard carrier variants.
These phones are both factory unlocked with dual SIM card support. Since they're international devices, they have an Exynos 8895 Octa in place of the Snapdragon 835. All the other specs are the same with 4GB of RAM, 64GB of storage, and a 5.8-inch AMOLED (GS8) or 6.2-inch AMOLED (GS8+).You also have the option of getting a gold phone in this sale, which isn't available in the US for some reason.
You should have most of the LTE bands for the US, but they're missing a few useful ones like band 12. Shipping in the US is free, but the seller (based in Texas) will ship worldwide. There's no tax, unless you're also in Texas.