The real problem is that there is no real alternative, so they can charge whatever the market will allow.
Forums and the free alternatives like Gumtree sometimes don't deliver in the way eBay does.
As an example, I've just had a 500D to sell, decided to try AVForums over the weekend, not a peep (not even the lowballers), when I check how many people where viewing the camera classifieds at 8pm on Sunday (prime time IMO) there were 7 people!
I moved the ad here on Tuesday, and put it on Gumtree. I then dropped the price and invited offers (I would have been happy at £250), nothing here, and only 3 scammers wanting it posted to Nigeria at Gumtree.
I advertised it on eBay as a Buy it Now with offers just after 7pm on Thursday, it was sold around 9:30pm at full BIN price. Even after all the fees I will end up with £256.
As long as eBay are so successful, and have such a stranglehold on the market (give them their due, they do bring the buyers in), what is the option?