Need advice on e-commerce site security

I am currently developing my first shopping cart, (yep im a newb). It recieves a object from the merchant which states whether or not the tranaction was succesful. Depending on this variable the transaction is recorded in a database using BeginTranaction/EndTransaction and a thread lock. The pages also use a https protocol.

Are there other techniques I can use to create a more robust/secure transaction, e.g. overcoming simultaneous transactions by same user and hackers. 

Thanks in advance for any suggestions.

 
 
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