While looking for a battery replacement (and most other online purcases) I check several factors.  No matter where you decide to buy online make sure to follow these 3 rules and you should get what you are looking for at the right price.

1-  Make sure to read reviews about the company or seller you are purchasing from as well as reading reviews and details of the product you are buying.  Most websites are very good at keeping all ratings, reviews and feedback without much bias so take into consideration all details you care about. 

2-  Check multiple online sites as well as manufacturers site to make sure what you are purchasing is the exact fix.  This is a crucial step so having the part number or model number is needed.
Also make sure to Check the return policies of the seller or website to make sure that they allow you to return the item without any hassle in case something is wrong.