Crear un punto de venta con ASP.NET y SQL Server

Diseño de la base de datos
Será una base de datos relacional, llamada pventa, muy básica, con la siguiente estructura:
Base de Datos
Y el script para crear labase de datos seria:
USE [pventa]
GO
/****** Object: Table [dbo].[users] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[users](
[user_login] [nvarchar](50)NOT NULL,
[user_password] [nvarchar](250) NULL,
[nombre] [nvarchar](150) NULL,
[activo] [bit] NOT NULL,
[ventas] [bit] NOT NULL,
[reportes] [bit] NOT NULL,
[administrar] [bit] NOT NULL,
[fecha_registro] [datetime] NOT NULL,
CONSTRAINT [PK_users] PRIMARY KEY CLUSTERED
(
[user_login] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[unidad_medida] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[unidad_medida](
[id_unidad_medida] [smallint] NOT NULL,
[unidad_medida] [nvarchar](150) NULL,
CONSTRAINT [PK_unidad_medida] PRIMARY KEYCLUSTERED
(
[id_unidad_medida] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[productos] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[productos](
[id_producto][nvarchar](50) NOT NULL,
[producto] [nvarchar](250) NULL,
[id_unidad_medida] [smallint] NULL,
[p_compra] [float] NULL,
[p_venta] [float] NULL,
[existencia] [float] NULL,
CONSTRAINT [PK_productos] PRIMARY KEY CLUSTERED
(
[id_producto] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON[PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[venta] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[venta](
[id_venta] [int] IDENTITY(1,1) NOT NULL,
[fecha_registro] [datetime] NOT NULL,
[user_login] [nvarchar](50) NOT NULL,
CONSTRAINT [PK_venta] PRIMARY KEY CLUSTERED
(
[id_venta] ASC
)WITH(PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Table [dbo].[venta_detalle_tmp] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[venta_detalle_tmp](
[id_venta_detalle_tmp] [int] IDENTITY(1,1) NOT NULL,[id_producto] [nvarchar](50) NOT NULL,
[cantidad] [float] NOT NULL,
[user_login] [nvarchar](50) NOT NULL,
[p_venta] [float] NOT NULL,
CONSTRAINT [PK_venta_detalle_tmp] PRIMARY KEY CLUSTERED
(
[id_venta_detalle_tmp] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]GO
/****** Object: Table [dbo].[venta_detalle] Script Date: 09/08/2011 17:27:43 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[venta_detalle](
[id_venta_detalle] [int] IDENTITY(1,1) NOT NULL,
[id_venta] [int] NOT NULL,
[id_producto] [nvarchar](50) NOT NULL,
[cantidad] [float] NOT NULL,
[p_venta] [float] NOT NULL,
CONSTRAINT[PK_venta_detalle] PRIMARY KEY CLUSTERED
(
[id_venta_detalle] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
/****** Object: Default [DF_users_fecha_registro] Script Date: 09/08/2011 17:27:43 ******/
ALTER TABLE [dbo].[users] ADD CONSTRAINT [DF_users_fecha_registro] DEFAULT...