eBay has amazed me for a long time, simply because they do have a virtual monopoly on this type of business, and they have been immune to competition.
Considering the barriers to entry in this business are small, the reason for their success is volume. They were the first successful auction site on the web, people from all over the world post sell their products on there, therefore most buyers go to eBay first.
eBay has not hit the financial gouging point that sellers and buyers are unwilling to pay. Sellers get more eyes on their product on eBay, and buyers can almost always find what they are looking for. Until another company can match that, everyone’s default choice is eBay.
I remember a while ago trying to break my habit of eBay first for precisely the reason in the OP. They continue to take a bigger and bigger chunk of the transaction price. As a buyer, it’s usually charged to me as higher shipping prices, so sellers still have some recourse. As a buyer, where else do I go?
Give me a viable alternative to eBay and I’d drop them in a heartbeat. I hate that company for so many reasons. But they are the best at what they do, and until someone else comes up with a site that attracts the sellers to post their product, eBay will continue to eat into everyone’s profit margin.