Ebay still has the most eyeballs for secondhand merchandise.
That would be its biggest advantage over the myriad of wannabe sites. Amazon seems to be catching up in some categories, and there are probably some good specialty sites for niche items, again depending on what you sell. Craigslist is catching on too but I've found that most CL buyers are ebay sellers looking for inventory, at least in our area.
That's the long and short of ebay's advantages. 
The list of disadvantages would take more time than either of us has to write or read. High fees, buggy software, seller-hostile corporate culture, lack of customer support, few buyers and highly demanding sometimes scamming buyers ... the list goes on. The old (easy, fun) ebay is gone for good I'm afraid.
If you're looking to supplement your income or want to sell as a hobby, ebay would probably be great for you. For anything more, my advice would be to run far, far away and don't look back.
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