Yes – water stains are not pretty. Also, they may be a sign of an active leak plus a breeding ground for mold and mildew. Remeber – they are also easy to hide from potential buyers with the right decoration and staging.
Be sure to look behind that pile of cardboard boxes or suitcase in the corner. You should always ask the owner to move the furniture a few inches and shine a flashlight around. If the home has obvious red flags (an odd odor or visible wall cracks), it’s not unreasonable to request removing a large picture frame to take a peek at what’s behind it.
Popular way sellers will conceal the water stains is with a fresh coat of paint.
Be sure to ask the owner when they last painted. If it was a year ago, they’re probably not trying to hide water stains.