The only way that you will be able to set up an online shop for free is if you design the site yourself using a webtool like Dreamweaver. But then you will have to purchase some form of javascript to put in for your shopping basket. Obviously you will have to know what you are doing with that software.
The easiest way is to buy some shopping cart software but again somebody will have to construct the site for you using that software and they are going to want paying for their time the same as you do when you go to work.
The truth is that you don't get much free in this world and like everyone else we all have to pay just like you.