If I'd have known it was that easy I might have tried to buy a ST this year! I thought it would have been impossible to sell tickets when I'm not actually there to swap a card for cash.
They don't get any extra money for a ticket exchange since the tickets are already sold. It's a pretty cynical idea but who knows. I think it might just be to avoid sparse blocks, which would make stewarding inefficient.
Looks very clever! Do we know for sure that the amount listed is actually the amount remaining though? I was under the impression that they released the south stand tickets on a block-by-block basis to concentrate fans away from the away supporters.