But, many coins are hard to come by and the chance to get one now may prompt an eager collector to pay a bit more for the opportunity. Patient collectors who bid lower may miss the coin, but, on the other hand, may get it for less. Only some of the sellers are competent professionals. 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.