But your thinking about this all wrong.

You did not buy seat 23A (or what ever your seat number was) on the plane between X and Y. You bought a right to a passage b/w X and Y.

You cancelled your ticket and as others are saying, according to the T/C of your purchse you aren't due a refund.

So whether easyjet sold or did not sell seat 23A is moot because you didn't buy a place on a particular plane.