As you've found out, 5-seater and 4-seater rear seats are not interchangeable (the actual floors of the cars are different, with different reinforcements, and being extremely structural cannot be safely modified).
If they're cloth and standard seats, then the answer is sadly not very much at all, and even less if the middle belt is a lap-belt only. These are the ones that everyone else is trying to upgrade from. There may be an off-chance of someone with a 5-seater in the same or compatible colour wanting them to replace stained/torn seats in their own car at nominal money. But 5-seaters are only about 10% of the A2 population, so unfortunately it's not much of a hope.
If they're Sport seats (diagonal line pattern fabric and/or the fronts have deep bolsters and electric lumbar switches) then the entire set might fetch £100 in very good condition, but before now I've seen a worn set in need of cleaning hang about even at £40.
Edit: Just seen your photo - those are standard seats with the lap-belt in what looks like Swing grey (from an early car, pre-2002). You could offer them free for collection in the Market but to be brutally honest, you'll probably have to take them to the tip to get the space back - sorry