I don't want to troll here but maybe this is more a sign of the old X360s being unpopular (due to low reliability, noice, heat etc) more than a sign of the Slim being attractive?
I always had the impression that X360s have terrible trade in value (rufly $100), although I only am familiar with the Swedish second hand market in this case. Not only cos of reliability/heat/nice but also because many of those boxes are modded and/or banned from Xbox Live.







