Event serialization is used to prevent duplicate event counting. Suppose, if the Event serialization is set to "Record once per Visit" - and if a user refreshes a page 10 times within a minute, it will just be recorded once. You can also use a unique serial number to serialize events ("Using event ID").
Merchandising evars are used to correctly attribute the success factor. Suppose you search for shoes and add a Reebok shoe to the cart, and then you also search for jacket and add a Reebok jacket to the cart. Without merchandising evars, both Jacket and Shoe would be credited with the internal search of shoe. However, this isn't true.
With merchandising evars, the search term "shoe" will be credited to the "Reebok shoe" and the search term "jacket" will be credited to "Reebok Jacket".