Except that there has to be a demand for it.  Ebay is huge, bitcoin is a drop in the bucket comparatively.  So that's what you can do as a customer: ask ebay sellers if they'd be willing to accept bitcoin as a payment.  It's becoming more and more well known, but I think a lot of people either a) still aren't aware of bitcoins or b) think it's a fad.

Personally, my bet on what would cause a huge catalyst for bitcoin growth would be business payment processors actually paying employees in bitcoins instead of dollars (or other local currencies in a non-US company).  I don't forsee this happening for a long time, personally, but I know I'd happily accept (some) payment in bitcoins if I were offered the option as an employee of a company.  As far as I know, the only company that has actually done this was The Internet Archive / archive.org.