Oh my god. That can't be for real. Those aren't prints, they're cheap posters i.e. a lithograph printed by a commercial printer (not fine art printer) with annotative text printed on the bottom and on very thin and cheap paper stock. They cost roughly $20 each from the Baltic Museum about 10 years ago. It boggles the mind that anyone would consider paying that much for them.
I believe people seeing his little doodle and signature (like in a book) for $5000 or something at auction recently (can't remember exactly). And it was addressed to that individual's name. That's why he doesn't sign anything anymore.