I am not recommending anything here because i dont use my site for a shop i use prestashop for that.  However if you go to the store here and do a search for shop and for cart you will find several plugins.  Again i know nothing of them, if they work, if they dont, but they are there. 

Hope that helps. 

Is one able to integrate prestashop with oxwall? We don't want our users to have to register with us several times

I personally have not seen a Oxwall -> Prestashop bridge yet, maybe there might be some bridges developed soon since this has been mentioned several times, maybe one of the more experience devs might do one.