I am looking and I have stumbled upon some open source software like osCommerce and Zen Cart
Do they offer a store front and how secure are they?
is there a lot of trouble shooting involved?
Will I have to be efficient in PHP?
I have an existing domain can I use one of these open source packages with it?
Yes, you need an existing domain and some web hosting and an SSL cert. Of the two, I’d recommed ZenCart. It’s based on OSCommerce but it’s been refined a bit by the Zen Cart team.