Selling any device with personal information on it, can be disastrous in the wrong hands, I am happy you are not one of those people.
Maybe an Email would suffice, or if possible, a call and inform them of performing a complete data wipe the next time they are planning on selling their phone.
With the criminals today, everything is fair game and an open season.
I would take note of the contact numbers, wipe the phone, and inform the seller of this, if only for good will reasons alone, and as you stated, he or she should be far more mindful of the potential problems that can arise from unintended personal information left on any device being sold or traded. Never a good or wise proposition for certain!