The Galaxy S10 is likely only a few months away, so we're starting to see even better deals on this year's model. Right now, you can get the dual-SIM variant of the Galaxy S9+ on eBay for just $599.99. That's roughly $120 less than the US unlocked model on Amazon.
This is the 'SM-G965F/DS' version, which has an Exynos 9810 processor, 6GB of memory, 64GB of storage (with microSD card support), a 3,500mAh battery, and a 6.2-inch 2960 x 1440 Super AMOLED screen. Since this is an international model, it won't work on CDMA carriers like Verizon and Sprint.
You can buy the phone from eBay at the source link below. Midnight Black, Titanium Gray, and Gold are available at the time of writing.