Gather round, cash-strapped gadget fans, and listen to this not-so-secret wisdom: high-end phones are ridiculously overpowered. Unless you're constantly live-streaming your breakfast while running NASA-level computations in the background, there's really no need for a 2.5GHz processor, a 2K screen, and 3GB of RAM. But high-end specs are still nice to have... so the frugal phone buyer waits until the next year's model rolls around, then snatches up last year's flagship on the cheap.
Case in point: eBay has the AT&T version of the Galaxy S5 on sale for $349.99. This is a phone that, just a few weeks ago, would have cost you $650-700 from any carrier without a contract. In fact, AT&T stores are still selling it for $570 while the new Galaxy S6 is on the shelf. The white model from eBay is a new 16GB version, and thanks to its Snapdragon 801 processor, 2GB of RAM, 1080p 5.1" screen, and 16MP rear camera, it's still a pretty great piece of hardware.
Since AT&T has near-parity with international GSM and LTE bands, this phone should work in most countries, though trying to bring it to other carriers in the US could be problematic. If you're interested, the seller is shipping to the US, Canada, Japan, Australia, New Zealand, and Europe (but not Cyprus, Greece, or Ukraine). Economy shipping inside the US is free, and unless you live in California, you won't have to pay sales tax, either.