I found this for sale on a website (will provide link below). Do you know if this is a copy of a genuine existing Gohonzon or not? I look forward to hearing what you think.
No, I do not think so. The linked Mandala looks like it might a calligraphy artist’s rendition of Nichiren’s Koan Era 10 worlds Great Mandalas. My understanding is that all of the publicly known and authenticated Mandala Gohonzons are published in the Gohonzon Shu. So, if there are more out there; they are either not public or of dubious origin.