Some eBay traders might have purchased 'bankrupt stock' of legitimate Duracell batteries.
Others might have purchased short-dated stock (because batteries do come with 'use by' dates).
However many will be selling fakes. Simply put 'fake duracell' into Google to see what I mean!
Here's the first, of many, links that come up:
https://thecounterfeitreport.com/product/106/Coppertop-AA---AAA-Batteries.html
However I've found that the alkaline batteries from pound shops (and similar 'discount stores') are usually every bit as good as Duracell anyway. (NB: That ONLY applies to
alkaline batteries. The zinc chloride batteries from pound shops are usually total rubbish!).
For example, give these a try:
http://www.poundland.co.uk/sony-aa-alkaline-batteries-4-pack