Originally Posted by TMOliver

I suspect you're suffering under a grave misapprehension/misconception shared by the OP who wants to buy a "seat". You're not buying a seat, but "Carriage" between two points (in the class and form described in the ticket you purchased, subject to a contract of conveyance described somewhere in small print). Simply "selecting" a particular seat provides you with no right by possession or any specific claim to same.

Some of us recall traveling on flights in which FAs and aircrew required pax to change seats, even to crowd together to prevent weight & balance issues. Certainly, an entire category of "special" travelers and those who accompany them might cause an FA to "tell" (not ask) you to move, hopefully to an equivalently comfortable seat, but may be a forward aisle to a middle seat far back in the same "cabin".

Compensation? Maybe a little, free drinks or trifles, but don't wait breathlessly for a refund.

IDB? You've got to be kidding. If in the the "Captain's" (Yes, the same general powers as the captain of a sea-going vessel) eyes, the seat which you selected and in which you're seated is most suitable for another purpose, register a complaint if troubled, but "Bow your neck", and the gendarmes will be in the cabin, hauling your not very smart carcass back to the terminal, subject to substantial mistreatment and perhaps an appearance before the bar of justice, unlikely, even with a good lawyer, to be sympathetic to the grave mishandling which you believe you have been accorded.