A list holds values in order: sales = [420, 515, 380]. Positions start at 0, so sales[0] is the first item and sales[-1] the last. A slice takes a run of items: sales[1:3] gives positions 1 and 2, because a slice stops before its end. Leave out a side to go to the edge: sales[:5] is the first five.
Lists change in place. .append(x) adds one item to the end, and .extend(other) adds every item of another list. len(), sum(), min() and max() work on a whole list, and x in sales checks whether a value is there.
There are two ways to sort. sorted(sales) returns a new sorted list. sales.sort() sorts the list itself and returns None, so ranked = sales.sort() leaves you holding None instead of a list. Add reverse=True to either one to sort from highest to lowest.