It's pretty much the only place to go for parts that are hard to find. (for example, most of the places that claimed to have my x60s battery in fact did not. the ebay seller, thanks to the ebay reputation system, did. I usually go to ebay over unknown resellers on the 'net simply because if an ebay seller says they have part X, they usually do.)
the other major trend is that both search and social distribution have substantially eroded eBay's original network effect as a destination for buyers & sellers... now both search (via SEM) and social (via FB & other networks) are also able to acquire & influence buyers.
anyway, i probably rag on eBay a little too much since i used to work there (via PayPal), but they have slowed down on the innovation quite a bit. that used to be offset by them buying innovative services, but that has also slowed down in recent years.
Sure, you can buy shit from your friends on facebook; I do that without an online social network, through mailing lists and other informal networks. But I don't think that really competes with ebay in any meaningful way.
craigslist is not in any great danger, but they also aren't innovating much... and even they too face competition from social platforms eventually if they keep standing still.
The whole point of ebay is that I can pay 'some guy wants to get rid of it' prices, and still get exactly what I want, even if what I want is kindof weird.
I am not sure that a social platform is best for selling things; I don't care what you look like, your sexual orientation, or how much you love your dog. I want to know if you will send me the item after I pay for it, and I want to know if the item will be in the condition you said it was in.