The problem with non OEM is you don't necessarily know what you're going to get. The battery could be filled with soda pop for all you know. Obviously the safe strategy is to buy OEM. If you are tempted to buy non OEM, then it would be wise to only consider one from a manufacturer who has an established reputation (i.e. good reviews). Then measure risk/reward.
Re eBay (and Amazon for that matter), you have to be careful. Some sellers will claim their battery is OEM when it is not. Sometimes the battery in the listing photo and/or the battery you receive will look (almost) exactly like OEM, but it is not. It's not always easy to tell the difference. Sometimes the battery you receive will look different than the battery in the listing photo, which is a bait and switch, and you should almost certainly return it.