As Fietsbob said it looks like a classic top tube dent from a handlebar swinging around. This is common after minor crashes, and years ago it was common practice to wrap a pad and/or tape over this spot to prevent dents of this kind, or later to cover them and claim it was prevention.
Not commenting on the value of the bike, but the dent isn't anything to worry about, except that it should be reflected in a lower price.
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