Well, ebay is a two-edged sword.
Variety is huge but we can be led into wasting some money. When it comes to buying coins, a clear pic does wonders as the seller's description can be very misguiding. S&H is one thing that really puts me off. Some sellers seem to use that as a source of extra income. Maybe a way to avoid ebay's fees.
One thing is for sure, ebay is changing the market for the average collector, at least in Portugal. I, for instance, bought a coin for €30 while the street price is around €80. It was a unique opportunity, but many coins are cheaper than the street price. How will the dealers deal with that?. I don't know.
ebay USA is even cheaper than the european ones, especially due to the exchange rate but I may get caught by the custom office and duties are heavy.

As a seller, I think ebay isn't very good for low cost items. Ebay+paypal take a nice chunk out of the final price but not accepting paypal reduces the number of potential clients, thus things will sell for less. It's some sort of catch-22 situation.