ALWAYS check manufacture date of the battery, you purchase!
I have put my own and Izzy‘s answer on Android Stack Exchange to formulate this check-list or rather list of importance for things that you should consider when purchasing a new battery.
tl;dr: Li-Ion batteries stored in store or warehouse without cyclic re-charging (which most likely never happens in these scenarios) are completely dead after 20 months at most. Thus, you should never, ever purchase any kind of battery that is two years old or more. No matter what seller says.
And that above is actually the best case scenario! Because with 5-10% self-discharge rate your battery will go to 0% charge in less then a year and keeping it in this state will render it useless within probably next 3-6 months.
No, it brings completely no difference whether battery was taken out of original package and used in any device or not. Actually, even more — if seller claims that they have never unpacked 1+ years old battery then you may make yourself sure that it was never charged during this time. And that it is dead or near its death.
What is most important is the fact that if you get a battery that was not charged for a year or more then it will not be immediately dead. On contrary — you will be able to charge it up to around 100% and it will work just as the new one. But its performance, capacity and other parameters will be very weak and will weaken from week to week.
As an effect, it will not work for a year or two, but for a seven months. A month more than its warranty.
Let me repeat myself. Any Li-Ion battery — no matter where, when and for what device or purpose it was purchased — is worthless and will break soon (or is already dead) if it wasn’t charged up to 90-95% each and every six months at least.
You can always use another argument, if your seller or discussion partner is stupid enough to understand above tech blah-blah.
If production date wouldn’t be so extremely important for batteries then why would manufacturer even bother printing it on each and every item? They’re looking for cost reductions virtually anywhere and would print such information, if it wasn’t crucial to battery quality?