Klumpen0815: Would anyone here object to a rule stating that codes have to be redeemed in a certain time frame (4-7 days?) or will be given to someone else? This seems to be a popular method against retrading.
I wouldn't want this wonderful thread to be abused in any way.
I'd suggest a month, personally, except those that expire, for which the rule should likely be to find as many as we can who'd redeem them before they do, so if some are left hanging and the date's getting close (5-7 days away?), find someone else, and if it's still nothing then try a 3rd time the day before the expiration date.
But since currently the problem is more along the lines of not finding enough people for some of those even at one code per person, that's probably theoretical. May apply to Banished though.