How do you store the sold flag in your database ? Is it text ?
If it is text you need to wrap sold in single quotes like so:
$query_WADAitems = "SELECT ItemID, ItemCatID, ItemName, ItemShortDesc, ItemLongDesc, ItemPrice, ItemSKU, ItemThumb, ItemImage, ItemWeight, StockStatus FROM items WHERE ItemCatID = 13 AND StockStatus <> 'Sold' ORDER BY ItemPrice ASC";
Seeing you db table and knowing how you store this would help solve the issue if this is not the answer