While the West is just getting the Nintendo 3DS, which went on sale over the weekend, the console has been available in Japan since February 26. As evident by this story, used 3DS handhelds are not in short supply.
All of these used 3DS handhelds are available slightly cheaper than the price for new units.
Japanese retail blogs, like this, report that the handheld keeps being sold back to stores. The reason for this seem to be various - such as a lack of compelling games. But upcoming titles are bound to change that.
Also, there does seem to be a percentage of 3DS units that were purchased with the sole aim of reselling them at a high price. The market was flooded with used 3DS machines, and Japanese retailers did not pay a premium for the buy-backs.
Remember, this is only anecdotal, and we're in the infancy of the 3DS's lifespan. If the DS is anything to go buy, the 3DS should continue to pick up steam - regardless, if it already seems to be petering out.
『３DS本体』中古ショップに大量に置かれる [オレ的ゲーム情報]