Trade in at GameStop for DS Phat is only $30. Sometimes you get the $20 bonus for trading in a system, so that brings up the total to $50.
I sold mine on eBay for $90, but I got the high end of the average because mine is a complete set with original box, Nintendo DS strap, and the Metroid Prime demo. Usually DS Phat goes for $60 to $90 on eBay.
I sold mine because I bought a new DS Lite at Toys R Us when they had the special where you buy the DS Lite at regular price ($129) and you get a $20 gift card, so it's like getting the DS Lite for $109.