I would be interested in buying it unlocked as well. When did the S6 have a US version that was unlocked?
The U.S. has never had a Galaxy carrier free version. They are not available for the U.S. market with its fragmented networks, similar to China.
There are, however, unbranded, unlocked, Global models that are GSM and will not work on the CDMA networks of Verizon, Sprint, U.S. Cellular, etc; and may only be partially compatible with some of the U.S. GSM bandwidths, depending on the carrier. These Global models are without bloatware, rootable and updated directly by Samsung but you may not be able to access some of your carriers features that are baked in to the various U.S. carrier models.