The prices on the dual-SIM variants of both the Samsung S8 and the S8+ have been steadily dropping over the past few weeks on eBay, first to $670 and $770 and then to $640 and $728, respectively. Now, the same seller has brought down the price on either model by another $40, putting the S8 at a penny under 600 bucks, and the S8+ at just $689.99 — representing a total savings of around $150 compared to what you would find if you tried to buy the same phones elsewhere.
This seller has a pretty solid track record, with a 99.7% positive feedback rating and a top rated certificate from eBay, so there shouldn't be much reason to be concerned about where you're buying from. Additionally, since they ship from Texas, shipping is free within the United States, and there are no taxes to pay unless your address also happens to be located within the lone star state.
One thing to note is that these phones are international devices, meaning that they'll be sporting Exynos 8895 Octa chips instead of the Snapdragon 835 that come in the US versions. They also don't support all the frequencies that are used by US carriers, but that might not be a big deal for you or your particular carrier. Aside from that and the obvious fact that they can house two SIM cards, all other specs are identical to their American siblings. The phones come factory unlocked, so you're free to use them on the carrier you love the most (or dislike the least).