This is from an estate coin collection of late 70th and early 80th, the seller is not a coin flipper.  This coin has been sitting in the bank safe deposit box for the last 40+ years untouched.

This coin was last purchased at 9-5-1980

There are two varieties for the proof version of the Kennedy half dollar coin, which are differentiated by the appearance of the mint mark. The Type 1 version has filled “S” mint mark and is the more common. The Type 2 version has a clear “S” and is scarcer, with an estimated mintage of 428,000 pieces.