I would say gamestop, or bestbuy depending on the condition.
If you could sell it online I’d go with Ebay or Craigslist.
Gamestop isn’t going to give you your full moneys worth in cash, they give you store credit. If you choose cash, they only give you a percentage of what it’s worth.
Source:
http://www.boomz.net/where-would-i-go-to-sell-my-nintendo-3ds-for-cash/?utm_source=rss&utm_medium=rss&utm_campaign=where-would-i-go-to-sell-my-nintendo-3ds-for-cash