You're saving a little over $200 on that price--for a 2007, so it's a great deal for that bicycle. The Hardrock comp is probably recommended far more often than any other for a starter MTB. I think you're getting a great deal on that--assuming all is in working order. Are you able to test ride it? And yes, I would take it to a bicycle shop for a basic tuning--unless the current owner is like me
I have a very rigorous maintenence plan for my steeds and I do all of my own work. I've sold stuff too, and make sure it is clean, relubed and in flawless working order before I sell them. My only concern would be why the seller is selling his Hardrock so quickly (it's a 2007 afterall).