I am quite sure that if people were required to pay before they were able to shop, many would not bother.  I would not want my money tied up with eBay just in case I may want to buy something here.  If I want something I Google it; I check all the options, if each one of the sites wanted a bond, I would have money tied up all over the place.  Some people only bid on something every few months, some only once a year, some may buy once and never come back.  

The way to make more money here is to attract buyers, not to discourage them.