If you are only storing something simple like a SKU and quantity, and not modifying/replacing values, a text file will be extremely fast and simple. You can open a text file for append and just add lines on the end, then close the file. You could also use XML and serialization, but if there are just a couple of values then a plain text file format would be simplest and serialization will add development labor and has more potential for performance issues, deserializing, updating, then reserializing the object back to disk over and over.
A DB might be necessary if you want to have relational data, or if somehow multiple users/devices could access simultaneously, or if you need rowset operations for data modification. If you use any db it will also access the file system every time there is a new row, same as using a file.