I was recently queried on this subject by a delightful member of the opposite sex and consider it my civic, yea patriotic, duty to shed light on the matter.
Shopping occurs when women go into stores with absolutely no intention of buying anything. They are sort of keeping up with what is available should the fever hit them. It is a sort of la-de-dah event and can actually be covertly blended with buying needed things by the skilled practitioner. Shopping is going down each and every aisle even when you know you don't want or need anything from that aisle.
Buying is a manly activity based solely on need and dates back to the stone age. For example; me need shirt...me hunt shirt...me find shirt...me stalk shirt...me kill shirt...me bring shirt home. Today, in the civilized world, it is: me need shirt...me go to specific location shirts are known to be...me find okay shirt...me buy shirt...me bring shirt home...me wear shirt with new-shirt wrinkles to proudly proclaim "new shirt" and success as a hunter/buyer.
I hope this clears it up for you. :-)
---William Fortner 2010---