The nearest place to Meridian that I could find for horse auction is Mize. The most famous horse auction in Mississippi is 3rd Saturday Horse Auction. It is carried out at Meadows Livestock Sales, Mize. If you are interest in horse sale than you can also consult Meridian Stockyard, which is located in Meridian.
But you did not mentioned the proper location or area in which you are finding horse auctions!