My obvious answer is take it to a jewellers for a new battery. I am not sure which watch you are referring too as you have sold a few over the past month or two.
However if the item was bought as second hand (no pun intended) then the life expectancy of the battery may I guess vary. On average for my watch a battery lasts 18-24 months.
Ultimately when did you sell the watch? If the buyer has a real issue, then they need to open a case and return it to you, or if you offer to send him a tenner for new battery.
