For whatever it is worth:
When I saw the listing I sent a message asking if it was hard plastic or soft rubber. She answered very quickly that it was rubber. She asked what the difference was, so I sent an explanation of the distinction between kinkeshi and M.U.S.C.L.E. and some backstory on what makes BHS special. I was careful to state that BHS is only a big deal if it is made of M.U.S.C.L.E. material.
She thanked me and asked what she should price it at given the explanation I sent. She claimed that she inherited a bunch of MUSCLEs when her brother died and was trying to figure it all out.
I tried to give her some advice that included references to stuff sold on Japanese sites. I was very careful not to actually link to anything or say the wrong thing, given eBay's policies, but apparently I was not careful enough because eBay blocked my message.
I *think* she may have changed some of the titles of her auctions, because I don't remember them using the words "rubber" or "kinkeshi" before. I'm not certain though as I didn't look too hard at the others. As others have noted, she did use the word "kinnikuman" and knew the names of a couple of the characters, so there was some research done.
Thank you