Watch out for scams. They are incredibly widespread. Absolutely, positively, do not buy it sight unseen.
If the seller has the original install DVD then run Apple Hardware Test - insert the DVD, start the computer with the power button while holding the D key. Run the extended tests. They may take 30 minutes or more.
If the seller no longer has the original install DVD, get proof of ownership to determine it is not stolen. No proof, no sale. The seller should also unregister it from his Support Profile. No one else can.
A serious seller will have kept the original box, the sales receipt, all documentation, all included DVDs. A good MBP that meets all these requirements as well as those descibed in wjosten's link above is likely going to command a market price very close to a refurbished one from Apple. Refurbished ones from Apple command prices very close to new. The money you save may not be worth the inconvenience.
Used Macs that meet these requirements retain their value for a long time. The used Mac market is nothing like the used PC market. You can hardly give away a used PC. I have several I got for free.