I built an Oscommerce cart from scratch in 2002 lost it due to acheap host decision on my part, I really liked it and once I got the PHP part down it was a little easier and the boards were very helpful and modules were easily found ready to go, the one thing that turned me off about miva.
Once i lost the hole cart (yes I had it backed up but I was recovering from being very ill and needed my cart back in biz quickly) I turned to miva and have been very happy with it as I also moved to Westhost at the same time.
So if you want a turn key solution that modules are available, although costly will pay for themselves, I use shipworks and will jump into sync soon to integrate my quick books, I am currently growing out of doing all by hand.