I've been a long time user of eBay, and have also followed their business performance over the years. Their core marketplace growth has disappeared. Amazon surpasses eBay for the first time
Speculation ranges from high seller fees to high risk of fraud on eBay.
I think the reasoning is more simple: everyone else has caught or surpassed eBay.
For a long time (5+ years) eBay was pretty much the only place to find, purchase, and receive "stuff" online. If you received your item, and it was even close to the condition in which it was described, you as a buyer were thrilled. This model served eBay very well. As their marketplace grew, people started
Problem is, the eBay experience has not improved. The rest of the e-commerce world has. Consumers now get better selection, better prices, better service, and a more streamlined experience from the rest of the web than what eBay provides.
I just purchased a couple "Stuff and Scruff" thingys that hang over the back of the front seats of a car, providing some organization for our kids' stuff, and keeping the kids' shoes off the back of the front seats. I found it through Google at an e-commerce site that looks like a mom-n-pop shop or one run out of the seller's home. It was easier for me to find something like this through Google than through eBay - Google provides access to a wider selection. If users are finding products through Google, then sellers have to keep up. As the e-commerce world grows up, it is inevitable that others are going to hang their shingle on the web and offer goods directly, providing their own experience to customers, their own customer service, and their own promotions. Sellers used to use eBay as their channel; they've now figured out how to do it without the help of eBay.
What happens if your customers bypass you?