Ecommerce - Do-it-yourself or Pre-Packaged Solution???



I was wondering if someone had a useful suggestion in regards to makin a simple online store. (shopping cart,online transactions). I know there are a lot of software packages available out there on many hosts (ie Miva Merchant) and was wondering what the pros and cons of using one of these are vs. doing it yourself or getting someone else to build it for you. I'm sure theres a tonne of other code samples that could also be used for these purposes available online as well. Any past experiences using these packages etc??? Words of wisdom?? Pitfalls to be aware of??

Any links or resources for php ecommerce solutions would be greatly appreciated. (ie good starting points to learn about ecommerce sites).