Isn't it better if next buyer knows if the battery was changed to a new original battery? I would prefer to buy a used set with brand new original battery.
if phone was opened before to replace battery, not sure about the waterproofing capabilities... if it's done at apple store, they will replace the rubber seal and probably do a proper job.
i read that there are different types of battery replacement jobs
a) original apple - done at apple store or authorized repair stores
b) grade a - will show that battery health % but not original
b) other grades which does not even show battery health %
of course the price difference is big - original apple is almost twice that of even the grade a... so i wouldn't really trust a used set with brand new battery even if it shows 100%