In short, its worth plenty. As long as its not physically abused, macs retain their value much more then other laptops. I just bought a used mac book pro from amazon from an individual seller and it has been great. Amazon seemed the perfect avenue compared to Craiglist (too risky) and Ebay (untrusted). Here are some links to see what they are going for.
http://www.amazon.com/s/ref=sr_nr_scat_541966_ln?rh=n%3A541966%2Ck%3Amacbo ok+pro+17&keywords=macbook+pro+17&ie=UTF8&qid=1325345260&scn=541966&h=b579317908cffea45da6c6d30b84c713db9fe863