If the watch is not working after you have added a new battery, you can open a return on the grounds that the watch is faulty or not as described.
Private sellers don't have to accept returns if you just change your mind, but they do have to accept an item back if it doesn't work as they stated in the listing.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)
