It's helpful to remember that the reason aircraft fly is to make money for the airline's shareholders.
To answer some of the points below:
Having paid for the seat it's yours whether you fly or not ? By selling a seat already paid for is this not borderline or straight forward theft ?
The contract you enter into when you 'buy a seat' is a little more complex than it used to be, as there are now penalties for poor performance, but that aside, it doesn't reflect any ownership of or rights to a seat on the part of the traveller.
It might be worth mentioning that I have seen 186 tickets sold on an A320 which can carry only 180
Thats quite a low oversell. Probably the finest overbooking system, at least that I've known, was one called COBRA, Capacity Optimisation Bringing Revenue Advantage, which used very refined algorithms, supplemented by live research, to calculate the probable no-shows. The over-sell could be in excess of 15% quite easily.
Now with Easyjet they get the revenue at booking, do they not ? So they fly a paid for seat. So by overbooking they have resold a seat already paid for ?
Sales are revenue, costs are more-or-less fixed, so if you can increase revenue without increasing costs, the shareholders will be happy. Moreover, low-cost airlines sell the last few seats for several times the price of the first ones sold, so this extra revenue comes in out of proportion to the majority of the sales. No one in the airline management will worry about whether a seat is 'paid for' in the sense above.
The things to get right about over-selling are that it must be advantageous in increasing revenue, and the costs of dealing with it when it goes wrong, such as compensating bumped-off passengers, and dealing with their eventual travel, must be less than the additional revenue, and the reputational harm must be acceptable.
And without over-booking, your tickets would all cost more.