It would be generous to inform them. If I was the seller, I would want to be informed. They might, however, tell you they disagree or ignore you. Likely they have an expert who has looked at the coin and they might think you are just uninformed.
While you are welcome to advise them it is similar to coins on the
fake coin report, please do not attribute any opinions to this discussion. We only give opinions on coins here. We do not coordinate informing sellers or discuss sellers. It can never be
FORVM or
FORVM members doing something to a seller. That could ruin our relationships with other dealers and soil the reputation of
FORVM.