Absolutely. The best way to warn the public about these scoundrels is to leave negative feedback. You cannot leave negative feedback unless you actually buy something.
If you are a
fraud victim and you are trying to get your
money back, you cannot really leave negative feedback before you get the refund. And if you do get your
money back, it may seem wrong to leave negative feedback after that.
You do everyone on the
eBay and numismatic communities a favor by getting these coins off the market, getting them into black cabinets where they can be studied, and giving negative feedback to those who are perpetrating
fraud.