Complex Stored Procedure, Plz Help

I created a site that will link the buyer and the seller together (think of ebay) for example:
Bob goes to my site to post an item he wants to sell (lets say a cell phone)
Steve goes to my site looking for a cell phone and he purchased Bob€™s cell phone (transaction is done on Paypal€™s site).
I have to make sure only one person check the same item at one time. For example
Steve and Mike are looking for a cell phone; Steve and Mike like the same cell phone. If Steve clicked on buy now Mike should not be able to check out the same item/or make mike wait to see if the Steve going to processed with his payment

after a lot of research i should do the check on the data layer not in the UI. so i need help to created a stored procedure that will do the following (do you think this is a good idea)

Case 1
1- Check if there is an order for this item in table checkout lets say item number is 45

Checkout Table
Item Number Email isComplete DTS


2- if there is no order ,a row should be created in checkout table
insert into checkout values (@itemnumber,@email,false,getdate())
Item Number Email isComplete DTS
45 Steve@hotmail.com false 04/12/2008 1:13:45

the stored procedure should return €œtrue€? at this point

3- whenever the column isComplete updated to true by another storedprocedure the row should be deleted
Delete from checkout where Item Number =45

Case2

1- Check if there is an order for this item in table checkout
2- If yes I need to check if it is => 20 minutes if true also the column should be updated with the newest date then it should return true
If <20 return true;


here are my stored procedure parameters




SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER ON

GO

CREATE PROCEDURE <Procedure_Name, sysname, ProcedureName>

@ProductID int,

@BuyerEmail varchar(200)

AS

BEGIN

declare @DTS as datetime

declare @IsComplete as bit

select @DTS = getdate();



END

GO


Thanks