In reply to andy:
Yes, I was going to post this. So it only makes sense where the fare is higher than the change fee, £35 each way, and the person offering the flight for sale is charging less than they paid for it. The saving will be the difference between what it cost and what the OP is charging.
The reason for this is to stop touts buying up flights and flogging them on, by the way, not just to be customer-unfriendly. Sometimes I wish it was true of gig tickets.
Neil
Post edited at 16:20