Estimating Price Elasticity from sales
In this kind of cases, I think you can not use a simple OLS estimation because of endogeneity of your variables. 

Probably, there would be causality between prices and sales data, this means that sales are influenced by prices but also prices are influenced by sales. So, in this case, in order to do a non-biaised estimation, you should use instrumental variables in order to avoid endogeneity issues between your variables.