Why Ebay? There are plenty of honest, reliable suppliers of rough who have been in business for years. Many of them are USFG members. It’s probably not appropriate here, but send me a message and I’ll get you a list of suggestions.
The topaz you saw is probably natural, but the color is a consequence of irradiation and heating. I don’t believe there is any synthetic topaz produced for sale, though it’s been reported as a laboratory curiosity.