Hi,
then your next step is to find out how you are going to save the user details between sessions. You can use player prefs, but I suspect it will quickly get messy. Have you investigated ways of saving data ( like frameworks on the asset store etc etc).
In terms of logic, there is nothing to it really.
-- keep track of how much money the user have ( a global var will do )
-- present the user with a list of items to purchase ( arrayList, or XmlMaker)
-- let the user pick an item, and deduce the price from the user money, add that item in the user list of purchased item.
Does that sound feasible?
bye,
Jean