My sons and I have three season tickets. One lives in London, one lives in Sheffield. They normally get to games but when they can't, it can sometimes be short notice when they let me know.
So there have been a couple of times this season where we've had an empty seat - and once we had two. I offered a spare to about half a dozen people for the Everton game and had no takers (it was fucking freezing that day mind).
To be honest, I've already paid for it, won't miss the money and haven't got a clue how to put spare seats on the exchange. Also, three seats in the Trent End probably take up the space of two seats in a building regs compliant new stand, so I don't mind anyway that some hairy-arsed stranger isn't squeezing in next to me.