This is a document which is required in certain countries or for certain commodities (such as pharmaceuticals), certifying that the specified imported goods are normally and freely sold in the exporting country's open markets and are approved for export.
Certificate of free sale is sometimes referred to as a "Certificate for Export" or "Certificate to Foreign Governments". This certificate is often required by some foreign governments. The certificate is evidence that goods, such as food or cosmetics are sold or distributed legally in the open market.
More information here: http://www.lachamber.com/webpage-directory/global-initiatives/initiatives-certificatesoffreesale/
Here's for the Netherlands - You may need a free-sale certificate (FSC) if you export food products, non-food products, animal feed or medical devices to non-EU/non-EEA countries. This applies among others to food and non-food products, animal feed, medical devices, crop protection agents and biocides. All FSC products must comply with European law and must be marketable without any restrictions within the EU.
To stay in touch with the information you need and what to do properly,check this out http://www.answersforbusiness.nl/regulation/free-sale-certificate