Well I'd hazard a guess, an advantage for eBay/Alibaba would be in freeing up resources currently commited to solving complaints/fraud. At least to some extent, thereby raising trust in their operations to a greater level.

Truth be told they can copy the tech used in Bitbay, however sometimes its just easier to buy out or partner with existing established tech (which Bitbay aspires to be), especially when you got access to capital as much as eBay/Alibaba have.