The important question here is the use of the word "and." If the items in the list are not units, the comma makes more sense before the "and."
I bought apples, bananas, pears, and grapes.
You do not want it to sound as if the pears automatically appear with the grapes, as it were.
However, if the last two items are a unit, you do not want to separate them.
I saw the Beatles, The Rolling Stones, and Sonny and Cher.
You do not want to separate Sonny and Cher because the "and" is part of the term. Ham and eggs, salt and pepper, Laurel and Hardy are other examples.
