That's what i want to know. Few sellers with reasonably good reputation are giving guarantee that they have 100% original new battery.
100% original new battery is hard to find for outdated phones locally, which is one of the reasons I never recommend outdated flagships. Even if you get sealed and trouble free set, awful battery timings due to fake batteries and outdated software will ruin the user experience anyway.
Latest apps are better optimised for latest Android versions, so apps are also going crash (causing battery drain) more often in outdated Android versions.