Update: The deal is back, from a different seller - head here to get it.
No one wants to drop $700-800 on an unlocked smartphone, but that might be your only option if you want the latest and greatest sans contract. In the case of the new Galaxy S4, eBay has been offering some compelling deals culminating in today's $599.99 offer. It's a heck of a deal!
The i9500 is the unlocked international variant of the 16GB GS4, meaning it will not work on LTE networks. You're limited to HSPA+ 850 / 900 / 1900 / 2100. That also means Verizon and Sprint customers are out of luck here. The i9500 runs on the Exynos 5 Octa 5410 ARM chip rather than the Snapdragon 600 found in the US versions.
Other than these internal differences, this is mostly the same GS4 you've been seeing around. It runs on Android 4.2.2 with less crapware because it's not carrier-branded. Updates should be a little faster too. Standard shipping in the US is free, but international buyers could get hit with up to $100 in additional charges. Keep an eye on that.