security while building online store

Hi, I was checking on several php/mysql based on line store, free and commercial, but didn't find any that fits the best for us. Looks like, it would be the best to develop one. To make a online store that works fine - it shouldn't be such a big problem. To make a SECURE ad SAFE online store - that's the tough one form me. Before I even start building the store, can somebody point usual "traps" and problems, and security problems that usually happen, some advices, any advices? Thanks for any help! -afan