Help required updating Master worksheet from a sales Worksheet

I have a Master Spreadsheet, with only about 17 columns of data, i get another sales spreadsheet every week with about 15 variables, from which i need to pick 4 Variables and update the Master. The Unique Field being Serial number , so if the Sale is already on the Master sheet ignore,else insert the three fields in the master
Anyone have an idea how i can Automate this?? copy of Master and sales sheet are attached