EDIT: see http://techcrunch.com/2015/04/10/as-vertical-marketplaces-ri...
- Arguably, there was something of a fad aspect to eBay's initial rise. Yes, there were serious buyers and sellers but I also suspect it was something of a game for a lot of people who subsequently moved on to do other things.
- In general, my sense is that buying/selling used goods is overall something that makes less sense today. That's a very broad brush statement but I've seen this directly in some areas with some local sports stores, for example, getting out of selling used gear.
- The overall broadening on the Internet population tends to mean that anywhere scammers can scam, they will. This is harder to effectively protect against in the type of market eBay originally focused on (one-off sales of pre-owned goods that were often not standard items).
And I haven't bought anything through their traditional auction format in ages either. I don't really collect stuff and I'm not much inclined to do the auction thing when a few clicks at Amazon or Newegg will get me almost as good a deal (at minimum) for electronics for less hassle and risk.
I love eBay as a buyer though, especially in Canada. There are still a ton of random things on there that really can't be found (at least not nearly as easily) anywhere else.
Unfortunately this only really works for items already listed on Amazon - otherwise you need to create new listings.