Try

CREATE PROCEDURE My1stSProc
@ListParam AS varchar(100)
AS
DECLARE @Sql nvarchar(4000)
SET @Sql = 'SELECT ItemID, ItemDesc  FROM dbo.ItemMaster' + 
  ' WHERE ItemID IN (' + @ListParam + ')' 
EXECUTE(@Sql)

>CREATE PROCEDURE My1stSProc
>@List char(100)
>as
>
>SELECT ItemID, ItemDesc
>FROM dbo.ItemMaster
>Where ItemID in (@List)
>GO