Many are competent amateurs, but many are incompetent amateurs and some are thieves. The major on-line auction site is eBay. More sellers sell more coins there, and more buyers look for coins there. Let's face it, many eBay sellers do not know what they have. Also, some small number know they have fakes and hope a gullible buyer will pay for them.
Fakes are a growing problem with on-line auctions.
This was not so a mere two years ago, but it is now. The potential for profit is obvious. This is happening more and more. But that leaves many misidentified. I wish I could say most of the reproductions were labeled as such.