incrediBILL - 11:47 pm on May 4, 2013 (gmt 0)
Having written commercial ecommerce software for 8 years as my job - DON'T DO IT FROM SCRATCH!
There are so many issues and gotchas, security stuff that you'll run into that starting with something already proven and tested is the only way to go. I'd look over the cart functions with a fine-toothed comb, esp. the payment gateway integration and make sure those CCs are 100% secure, never stored in a session file, yada yada.
As a matter of fact, consider making your run-time site encrypted with Zend or IonCube if it's PHP so that in the event you ever get hacked, hackers can't drop a line of code in your store to send plain text CCs which I've seen done more than once.
FWIW, having the store encrypted keeps nosy people out of your site on a shared server in case they manage to escalate their privileges, or happen to work for the host as not all IT people are so trustworthy.
Never can be too safe.