you could check ebay or you could take it to somewhere like GameStation and they would tell you how much they would give you (in store credit) for it. It would be a lot less hassle that to sell it on ebay.
To be honest you might be better just to spend you money on a new game as the lite isnt that much better (apart from the size and backlight on the screen) i spose itdepends how much you play it and how much or a serious gamer you are.
I saw an advert for the new colours of the ds lite the other day in Game, and it said that you could trade in your old ds lite and pay �49 , or trade in your old ds and pay �69 for the new ds lite.