The 1931-S is the second lowest mintage in the entire Lincoln cent series. The mintage of the 1931-S Linoln cents were revealed soon after they were struck and those who knew about the low mintage, would usually hoard the coins.
Consequently, since the early 1930's the coins became very scarce in circulation. In fact, during that time some collectors reported finding a 1909-S V.D.B. Lincoln cent in circulation, but they never found a 1931-S.
Because of it's low mintage and all of the hoarding, the 1931-S Lincoln cent can be purchased in uncirculated grades fairly easy. The monetary spread between grades of this coin is very thin. In circulated grades it trades at very close premiums and in MS60 to MS64 grade examples can also be purchased for just about the same price.
Walter Breen indicates that an individual by the name of Maurice Sharlack single handedly acquired a hoard of 200,000 Uncirculated 1931-S Lincoln cents.
Circulated coins to MS64 coins can be purchased without much trouble. In MS65 grades, they are less common but still obtainable. The ultimate grade is MS66 and possibly no more than 100 examples exist in this grade or higher. Many of the MS66’s are of average quality. Finding an MS66 of high end quality or better, will definitely require a lot of searching. As of this writing, there are no MS67’s in existence and there probably never will be.
According to a notice in the June 1934 issue of The Numismatist (p. 416), collectors could still purchase Uncirculated 1931-S Cents for "the face value of the coins and an amount sufficient to cover the mail charrges by first-class mail."