You can't trade JUST you DSi system in for a 3DS system anywhere.
Have a look at videogameplaza, maybe you will get some useful information.
No. Trading in a used DS will get you credit toward a DSi (somewhere around $70), but it will not pay for the entire thing.
No, but you can trade in your old DSi for credit toward a 3DS. So if you trade it in, you'll save 50-100 dollars on your 3DS.
You cannot trade in your DS for a DSi without paying extra money, due to the DSi being more advanced and up to date. Also, the condition of the item can control how much it is worth. If you have a...