Hello Andy and welcome to the forum, on the picture showing the back of the watch it reads Timex H cells these are the same as silver oxide 386, SG12 or SR43 batteries which you can find everywhere. I’d try a couple of cheap AG12 batteries first though to see if the watch works OK then I’d recommend changing them to the silver oxide batteries as they’ll last much longer than the alkaline AG12’s they’re a little bit more expensive but worth it.
Your watch looks to be in very good condition plus the logo’s intact so if it fully works then $50 is a very good price for a Timex LED, I’ve paid similar amounts for worse condition watches just to get a working module.
Hope this helps...