You'd think that the club have the infrastructure to do the whole thing in house.
If you can't go, you tell the club, they simply add your seat back into the available pot (much as they already do with Cup games for STH to secure their own seat?). If it sells, at the standard single ticket price for that game, that amount (less a minimal admin fee, a couple of quid?) is then credited to your account, perhaps paying off your installment or even going into credit towards your next ST, or cup ticket, etc?
Club keeps full control, fans get a fair deal, some external company doesn't profit and the cockwombles profiteering off fellow blues, lose.