http://beta.glitch.com/forum/general/4384/
that topic pretty much speaks for itself. despite there being restrictions on this in the terms of service there is very little that can be done to regulate this.
ultra rare items should be trophies or achievements instead. non-transferrable stuff. even then there will be account trading but this should mitigate a decent chunk of the market.
all players ought to be able earn all items on their own, with no special stuff only available for short periods of time. if some items are only available seasonally, that works. if it were possible to earn a glitchmas yeti again this glitchmas, i think that would be enough to take the bottom out from under this.