... Where is that demand going to come from ? eBay has an operating income of over $3 billion dollars...
Taking market share off them will be like pulling teeth because the nature of auction markets is such that the bigger the audience for your auction, the higher the price you can sell your stuff at.
In 2013, EBay generated $7bn in net revenue (i.e. fees from selling). The total gross value of goods sold was $77bn (from 550m listings).
These simple (and easy to check) facts shows you can't really be trusted.