Local, non-chain game stores often have used 3DS consoles for, like, $120.
Do note that the closer we get to Pokemon X & Y the higher that price will go. Your alternative would be one of the sales that have been occurring regular lately, but again, the closer we get to Pokemon X & Y the less frequently those sales will occur.
This is because the reason 3DS's are going so cheaply was due to the low demand lately due to the low amount of "must-have" games for the past year or so. They released some good games in 2012, but nothing that really moved consoles. However, we're now in the middle of a 3DS "rebirth", if you will, so keep your eyes open for a good deal.