When you say honest i dont think the seller is doing anything wrong personally.
There are loads of them (fake coins) and readily available ,unfortunately its the people who resell them as genuine that are the problem and they will always find them.
Not every collector wants to spend alot of money on the genuine coin so maybe fills a gap with a copy.
If its who i think it is he has been or was a dealer for years and if not there are boxes of decimal sets turn up regular so nothing untoward there.
Only my opinion
Pete.