There are certain things that could be be producing wrong results:
* Some variables in the model appear to be non-significant, that is, these measures don't help you explain sales. Before adjusting the model, try analyzing the relationships with scatter plots and correlations.
* The Response Variable may have some skew (when dealing with money it is very common), you should analyze it first.
* Certain assumptions may not be met in your model. The most important are normality in residuals and common variance across the observations. Those assumptions must be checked graphically.
* Since your response variable seems to be measured over time, your data may be autocorrelated, which will cause troubles in an Ordinary Least Squares Regression Model.
As you can see, your data appears to be a little tricky. Try doing a deeper exploratory analysis and I would suggest you to perform a Robust Regression Model.