One of the major assumptions in much of economic theory, is that you have a bunch of people, each with a collection of different items, and each having different opinions as to how good or bad any single item is.
An example, is a group of kids in a school playground who have each been given different things in their packed lunch by their parents. One kid loves eating cheese (but was given a sausage roll by his mum), while the other loves sausage rolls (but was given a piece of cheese).
Economics then says, that the people in these groups then choose to swap, so that they get a mutually beneficial result.
Or in this case, the kids with the cheese and the sausge roll do a swap, so that each gets to eat the food they like for lunch.
Once easyJet implement allocated seating, will there be anything to stop people openly swapping seats before departure ?