How can I display multiple column records in the same row?
You can apply the GROUP_CONCAT function on top of your existing query like so:
SELECT Trader, GROUP_CONCAT(Items, ',') AS Items
FROM 
(
    select NPC_Traders.Nickname as Trader, Classes.Class as Items
    from NPC_Traders, Classes
    where Items IN (
               select npc.Tradeables
               from NPC_Traders npc 
               where npc.Tradeables LIKE '%1%' OR
                     npc.Tradeables LIKE '%2%' OR
                     npc.Tradeables LIKE '%3%' OR
                                    npc.Tradeables LIKE '%4%')
) AS Subquery