If it was a cosmetic situation, I would get more suspicious for accidents/drops etc., but for a chain replacement, I would consider that as an upgrade, since most of the after-market chains (lifetime warranty) are better quality than the stock chain (20,000km warranty) of the Ninja 250.
If the bike has been ridden in the rain, and not had the stock chain lubricated frequent enough and taken care of, then obviously the owner could go for the new chain and a new set of sprockets.
Keep in mind, this is the opinion of the noob tho
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