Here I can buy new 500GB drives for under 50 bucks delivered to my mail box. Since I want such things to be reliable, the small price would have to be say less than 1/2 the price of new.
I am able to buy the specified used drive for small price. Disk is 4 years old. What worries me is that disk have more than 30 000 powers on hours count. I read on this page that total estimated life time of HDD is 43 000 hours. What do you think, is it worth buying?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic