The 100% certain way to get a legit and safe battery is to buy it from the manufacturer (Acer in this case).
Of course, downside is they are much more expensive.
Unfortunately, there is really no way to tell when buying from ebay. People lie all the time on ebay, even sellers with very good ratings, and when it comes to batteries, China makes a lot of cheap cells (Japan and Korea make the higher quality cells usually used in official batteries). Unfortunately, there is no way to tell, unless you are a researcher in battery technology, and have very expensive lab equipment. Even opening the battery and checking cells inside is useless nowadays thanks to the proliferation of counterfeit cells.
They may or may not perform the same as high quality cells, but will definitely be lacking in protection mechanisms, etc.
