Dear god do not buy that chip. It may be working now, but who knows in a day, a week or a year. Also, who knows how he tested it? Booting into Windows? Running a couple applications? Stress testing with Prime95?
Chances are that chip is not functioning at the same capacity as an undamaged chip, and if something goes wrong with it, you're left holding the bag. Then you're out the $100 for the chip and have to buy one at full price anyway.