Dude, I'm not sure what the other guys on here will say but make sure you take it for a ride before you make any kind of deal. I would start it, take it for a ride for at least 15 to 20 minutes. Once you bring it back leave it running for a while. Check to see if there are any leaks and if there are any cracks in the case, especially around the front sprocket area. Make sure that before you take it for a ride that he has all the fluids FULL, including the gas. It might seem like making sure the gas being full is a little much but trust me it's not. When I brought mine home I didn't check to see how much gas was in it. Everything else was great but when I brought it home and filled it up, gas was leaking everywhere. The guy put in screws under the seat to secure the gas tank cover better and he punctured the gas tank. It didn't take much to fix but it's still something that pissed me off once I got it home.
Also take into consideration the money you are going to have to put into it for the necessary things it needs to run and perform properly. This will give you a little bit of an upper hand when it comes to negotiating a price.