In most cameras and smartphones the back light is integral to the display assembly, and if it's possible to separate them in order to replace one but not the other it would probably be classed as an expert repair.
That said, if you've got a broken LCD, you've nothing to loose by carefully trying to take it apart, in the process finding out how it fits together. With that knowledge you might be able to fit your old backlight to your new LCD. But if your old LCD is still just about serviceable you might not want to take the risk. On balance I'd be inclined to look at the eBay seller's returns policy, and if possible send it back and find another complete display from somewhere else.