Didn't we go over this with Captain Preferences a month or two ago?
While your deal doesn't sound nearly as fantastic/unbelievable as his, I guarantee you one of two things will happen:
1) It's a used 360 with no receipt and probably even refurbished so there will be no warranty.
2) It has been modded.
You're really better off just getting a new one.
-edit-
I should learn to read better, ignore point number 1. Still though, I wouldn't trust the seller and would just get a new one.