I used to sell valuable stuff ($2000+) on Ebay, back in the day, but Ebay has made a lot of changes over the years making the environment hostile for sellers. First, sellers can no longer leave negative feedback, but buyers can in every case, even if a seller cancels a transaction and attempts to refund the purchase. So, abusive buyers don't have any trail behind them.
It used to be that safer forms of payment could be used, such as a postal money order, but Ebay makes it difficult to use anything but Paypal. Paypal almost always rules against a seller in the event of any dispute. So, it seems quite a simple matter for a dishonest buyer to steal from a seller, especially a low-volume seller who does not have a legal department on staff. "I never received my item" is one possible lie, but a more cunning one is, "The item I received was not as described in the auction," which is impossible to disprove, and as noted above, Paypal always rules in favor of the buyer. So, as a seller, there is a chance that you will become a victim of theft that involves a significant amount of aggravation and humiliation added for good measure. Ebay suggests that you factor such losses into your business plan. It is like shoplifting, they say. Incidentally, if you are stolen from, you will also be charged Ebay and Paypal fees, increasing the amount of your loss.
I still sell on Ebay, but after a buyer blackmailed me over a $5 transaction, threatening to leave negative feedback unless I sent him free stuff, I opted to remove all auctions that are for items greater in value than $100. Buyers are acting in an unsavory manner, abusing the system to their advantage. Ebay doesn't care, because they still make money, no matter what happens to the seller.
The total amount of fees that Ebay charges is another matter. For me, they have been in the 16% to 18% range. Ebay also undercharges buyers on shipping costs on a pretty regular basis.
Overall, I think the usefulness of Ebay is not what it once was, and I would welcome a strong competitor. As far as I am concerned, Ebay is a good place to sell junk that would otherwise go into the garbage. I don't intend to sell anything valuable on Ebay ever again.