I am thinking of moving to ecommerce development. There is just so much out there so much so that I find myself at a loss as to where to start.

I do front end development but I understand backend equally well. Ideally I would like to find a suitable e-commerce system that I can work with, tweak and customise. So if a client needs an ecommerce site my task would be to build a custom interface and hook it up with this system. If some changes are necessary to make this work that would be done.

What are the best e-commerce systems out there? Open source an option?

Your thoughts are much appreciated, feel free to go on a tangent if you feel that may be helpful.