Marc, the Canadian who sells on eBay as wideangleman is, to be polite, a wild-eyed raver. He doesn't understand much about the lenses he offers or he lies shamelessly. I fear both may be true.
Do you know Bob Monaghan's MF site? He's posted a couple of articles there about hacking inexpensive 400 mm and longer "telephoto" lenses sold for 35 mm cameras. Many of them have cemented doublet achromats at the front that will form an image. And in fact some so-called tele lenses, e.g., the Century 500/5.6 Tele Athenar II I sold a couple of years ago, are simply a cemented achromat at the front of a long tube. Long focus, not telephoto at all.