I'm not sure if you have a Gamestop where you are, or some kind of similar game shop, but Ebay is always a good choice if your unit is like new and complete. I sold my original Phat DS on Ebay right when the lite model cam out, and I got about $130 for my complete system(original box, manuals, metroid demo ect.). I would imagine you could get a good amount if you are an ambasador member, and your unit is like new. You could probably get very close to retail(minus Ebay & Paypal fees of course)I have a 3ds and ive never played it.Wouldn't get much if i traded it in though so will hold on to it.I shall buy a vita though even though ill just barely be able to afford it.It's more my style than 3ds.Just traded in my 3DS and some games, and paid off the rest of my Vita pre-order. It's going to be a long week!
Right now Gamestop here in the states is running extra credit promotions, so they are a good alternative at times, but I personally refuse to sell them anything, so they can mark it up 80% after I leave.
Chris