There may be 2 differences
1. The quality of plastic. You would expect them to be similar, but they can even fail here. It could be a little tight fit for example
2. The inside material. Most canon ones have black felt (kills reflections), while ebay could be plain and reflective plastic. But then for example canon 85/1.8 hood is awful.
Do your research and make sure you buy the right one, don't just blindly click on the cheapest.
any idea what hood to get for a an 85mm f1.8 as that's what I'm looking for!