This depends a lot on your circumstances.  If you are a casual seller with a small range of items, GT or FB might be viable alternatives.  No Fees at all. Go nuts

However these platforms are not fit for purpose for larger sellers, or sellers with a large range of products.  No payment processing.  No Buy it now (with payment).  Endless message windows popping up, chasing payment and so on.  And of course, there is no order management interface, no listing automation, no inventory sync, no API connectivity.

When an alternative platform offers me an API based experience into my system (like Ebay), with inventory sync across platforms, payment processing, template based listing creation, custom field mapping, order sync with central platform and so on I'd happily consider them.  

Ebay is not perfect, but the fees are not unreasonable considering you have a ready made market to sell in.  Build and market your own website and see how much it costs you as a percentage of your sales.  You'll be begging for 13% let me tell you.