EBay seller 232tech currently is selling the international and unlocked variant of the Samsung Galaxy S6 for $529.99. This is a deep discount on the device which normally retails for $699.99.
The phone will work with ATT and ATT based MVNOs as it has all the necessary bands for LTE. It will also work on T-Mobile but you won't have full support for the network as it lacks T-Mobile's main band for HSPA+. It does contain bands 2 and 12 for LTE.