As with everything, there is the good and there is the bad (and sometimes it gets ugly [sorry, I couldn't resist]).
You could change coin seller with any other type of seller and the analogy is still valid. Please note that I use the term "seller" and not "dealer" inasmuch as, at least to me, they mean something different. By my definition a dealer, (in anything), is someone who earns his living by marketing and selling a specific genre of item AND has some sort of professional status and respectability. A seller is someone who has no professional status and is a casual purveyor of whatever he/she purports to sell.
Specifically as an example, speaking for myself, regarding coin dealers and admittedly applicable to other products, there are professionals whom I would trust implicitly on their word alone as to a description or authenticity. There are others I have so little confidence in, I wouldn't trust them if I examined the item myself in hand. Some of these dealers do not even have a presence on ebay. I'm sure that most of you have had experiences that you can apply these ruminations to.
Ebay, being a vast wasteland, has become a haven for charlatans, snake oil peddlers, and others of that ilk. Ebay, being a vast worldwide marketing venue is also home to some of the most respectable, honest, and classy people that I have come to know. Essentially it is a world within a world with its own broad spectrum of inhabitants. Unfortunately there is a miniscule amount of monitoring of sellers by ebay except where it affects their business model. It takes much effort on the part of potential buyers to convince ebay that an issue needs to be addressed, and more often than not, their actions are sadly inadequate and insufficient.
When all is said and done, however, class and honesty will rise to the top and as a result will bring repeat sales, which is the foundation and cornerstone (keystone) of a successful business. The fact that any business, whether online (by itself or part of a larger market venue) survives for a significant length of time is and of itself a testament to its integrity and that of its management. Ebay survives because 8t is a worldwide venue that still has good sellers on it that draw repeat business despite the bad sellers that draw the ire and complaints of buyers.
There is that old adage, "fool me once, shame on you; fool me twice, shame on me". You just don't buy from them again.
This diatribe is not focusing, accusing, or targeting any individual or business. It is merely an observation and commentary of the way things are. Simplistically, you can't have good without evil, nor can you have evil without good. They are the yin and yang of life.