The 1931-S is a frustrating date. Despite the low mintage, many of the coins are weakly struck, with mismatched die states. On your 1930-S topic, I mentioned the intentional obverse re-design for 1931-S, moving the ribbon from behind the last digit of the date because of concerns that it would merge with the last digit too easily on a date ending with a "1". Given the weak San Francisco strikes in 1931, the one-off re-design was a smart move.
Your coin has a mushy central strike with better peripheral details. A couple sharp bag marks on the braid and scratches on the buffalo's upper shoulder and flank are distracting, but not anywhere near enough to detail the coin. On the obverse, the date, LIBERTY, and first feather have good detail, but the braid and ribbon are somewhat weak. On the reverse, the horn, eye, right rear leg, loop of the tail, and lettering are sharp, but almost half of the hair is weak or missing on the shoulder and upper back. The fields look clean on both obverse and reverse.
Overall, wear is light, and confined to the usual high spots. For the date, this is a pretty solid coin. Balancing the strike weakness and wear, I see an EF-45 obverse and AU-50 reverse, with a blended grade of EF-45. At $30, you are spot on at EF-45. This is one of those coins that I more and more see being called an "AU slider" at shows. If that eventually generates a new pinpoint grade of EF-48 (and I think it will), this coin would earn that grade. But for now, the distance between EF-45 and AU-50 holds it to EF-45.
It's a nice coin, has better eye appeal than many of the 1931-S coins I see, and is well worth the price. You did well on this purchase.
