I would personally put it on EBay-it's wasy to use, & will attract a lot of interest. It's quite easy to get to grips with once you've registered (esp if you have a Paypal account so they can pay you as soon as the bid is one)
You could also try
www.cex.co.uk as they basically pay half (in cash) of the price that they sell 2nd hand for-if you take a credit note for later use, you wil get 2/3 of the price they sell second hand for-I've used them for years & found it a really good way to upgrade stuff.
Give me a shout if you want an more info on EBay.
HTH.
[Edit]-here's a link to the XBOX buy/sell page on CEX, which shows you how much they will pay you.
http://www1.cex.co.uk/product_list.a...&Submit=Search