I’ve bought OEM mobile phone batteries for various models and brands from 3rd party sellers on Amazon for years without any problems.
I make sure that I choose sellers with a lot of positive seller reviews and sales (preferably in the thousands), that battery is a genuine OEM, that the battery has a lot of real reviews (hundreds, if not thousands), and the battery reviews are at least 3.5 stars (with reasonable/acceptable reasons for downvotes in the reviews). Finally, I only buy if the 3rd party item is shipped by Amazon so I have Amazon’s backing in the case of any problems/disputes.
I tend to pay $6 to $9 per battery, always with free shipping (sometimes I can stack a slow shipping bonus because I have Prime). All of the batteries I’ve bought on Amazon have lasted as long, or longer, as the original battery and performed exactly the same.