You'd be best going on ebay. Macs, on ebay, get a good price. They'll charge for the auction of course, but you'll get a lot of people interested. The more people that look at it, the more $$$.$$ you will get. Also, you don't have to be worried about being scammed - both the buyer and seller are better protected.
If you do get the Alienware, I'd really advise getting the 4GB memory and 7200RPM HD option.