I bought my IPhone 6S on 22.06.2018, and with in 2 and half month not completed, the battery is draining quickly. I would like to get help from your good self on this concern.
Batteries do not last forever. This particular issue is neither the seller's fault nor eBay's fault, and they shouldn't remedy the problem.
However, batteries for the iPhone 6 are a major problem and Apple has acknowledged it ... they have a discounted repair program in place through the end of the year (sort of like a recall), where you can have the battery replaced:
