The paper considers the external validity of the growing set of literature that uses laboratory auctions to reveal consumers' willingness to pay for consumer goods, when the concerned goods are sold in retailing shops through posted prices procedures. Here, the quality of the parallel between the field and the lab crucially depends on whether being informed of the actual field price influences a consumer's willingness to pay for a good or not. We show that the elasticity of the WTP revision, according to the field price estimation error, is significant, positive and can be roughly approximate to one quarter of the error. We then discuss the normative implications of these results for future experiments aimed at eliciting private valuations through auctions. (A paraître in Experimental Economics)