ODO defines marketing to mean...
the action or business of promoting and selling products or services, including market research and advertising.
... which would make her usage of the word incorrect, as she isn't selling anything, but rather purchasing products (or services).
The American Marketing Association defines marketing as...
the activity, set of institutions, and processes for creating, communicating, delivering, and exchanging offerings that have value for customers, clients, partners, and society at large.
... which means that the money she is exchanging (something of value) could be going to her "client" (the supermarket). But that's quite a stretch. Based on the spirit of their definition, again, her usage is incorrect.
Merriam-Webster does have a definition that fits her usage...
the act or process of selling or purchasing in a market
... so there are still dictionaries out there that back up her usage. However, even MW goes on to push the definition more toward selling and promoting products, as opposed to purchasing them.
So, it is still "appropriate"? Most likely not, since most reputable sources are allowing that use of the word to go to the wayside.