mywit via eBay offers the refurbished Unlocked Apple iPhone 6s 64GB Verizon Smartphone in Space Gray for $589.99 with free shipping. That's the best price we've seen for an unlocked 64GB iPhone 6s in any condition. (It's also $210 under the best price we could find for a new unit.) It'll work on Verizon or GSM networks such as AT&T or T-Mobile.
Of note, no warranty information is provided.