You can get the international unlocked Samsung Galaxy S6 for $500 with free shipping in the USA right now on eBay in black, white, gold, and that horrific blue. It is brand new and should work on most GSM networks, including AT&T and T-Mobile in the USA. This is an extra $30 cheaper than the deal we found just a couple of weeks ago, not to mention the added benefit of color choice.
For comparison's sake, if you were to buy the AT&T equivalent (G920A), it would run you $685 before tax. Needless to say, this is a substantial discount. The seller describes it as brand new and sealed in its box, which supposedly comes directly from Samsung.
Speaking of, the G920F in the listing has full support for AT&T's network and should work without a hitch once you put a SIM in it. It is missing T-Mobile's main HSPA+ band, though, meaning you will likely be looking at 3G coverage when you don't have LTE as we explained in a previous deals post. Those looking to use it elsewhere in the world should check the item description to ensure it has the bands necessary for their local provider.
The seller isn't divulging the number available, but regardless of the exact amount these will probably go fast.