Its really hard to say, because as you have said asking gets little to no response on the forums. If the Tegus you have seen for sale are babies then their coloring goes through stages and changes as they grow. I have seen hugh "normal B&W" Tegus and small ones. I agree it could be some form of line breeding or a pitch to make a buck, but those are just my personal thoughts as I have never seen a "Chacoan Giant" in person.