Personally, I'd stick with a ready made application. Most popular systems have support forums and detailed documentation, and you can be assured that it's a well-coded, efficient piece of kit from experienced developers.

That's not to say you shouldn't code your own, but judging by your experience I would not recommend it.

These things can take a lot of time and trial and error. And being e-commerce, you really do need to know what you're doing, since you'll be dealing with secure data transfer at risk from hackers.