How to estimate empirically a demand curve
I think You will have to account for various factors other than price. What is your purpose in estimating the demand curve? 

This link will give you a good explanation on how to estimate demand curves if you are looking for simple exposition - assuming that sales and other factors have linear relationship. 

If you are good at econometrics, you may want to consider simultaneous equations models. It is likely that your single equation model will suffer from simultaneity and omitted variable bias along with identification problems.