How come nobody has asked him how much?
As for buying it used, assuming it was made within the last three years, you have the warranty. That said, make sure you can see it first and make sure it has not been modded. After that, turn it on and make sure it works. If he lets you, play it for 30 minutes.
Personally, I would go for a new one since it's the safest. I wouldn't worry about the RRoD, I would worry more about what the guy did with the console. You don't know if he modded it of if it did get the RRoD and he just used the towel trick as a temp fix until he could sell it.
Since you're coming on late in the generation, there are A LOT of great games available to you for VERY cheap. Just start with the early 360 games and work your way up. Most of the greats are $20-ish and buy the time you finish those, the current greats will be down in price.
-edit-
360 - assuming it's in good working condition, $100 at most.
Hard Drive - $20
Skate - $5
If you're paying more than $125 I wouldn't bother and would just get a new one. It will be much more, but then you get a brand new one and can get a 120 Gig hard drive.
