Sounds like he's just trying to get rid of it. Do you need it? What specific model number is it? It's probably some low-end craptop, to be honest.
How much is $100 worth to you? Based purely on price vs. the machine it would appear to be a decent deal. Of course, you have to ask, why is he trying to get rid of a decent machine for so little? Is he rolling in it to the extent that he can basically give it away? The fact it has no OS means that either the HDD has been wiped or replaced. It would be important to know which one, and why. So in summary, potentially this seems like a good deal but there are some important questions to ask about the machine first - and do you trust him? On a personal note, I'm not a big fan of HP machines. I've not used this particular model and I don't know much about the history of the line. Personally I wouldn't buy it unless I really needed a similar machine, for cheap, and for casual usage, and I was certain of what had happened with this machine and why it was being sold cheaply.
Is this new, or used? Going rate for these looks to be around $225 - $325. $100 without an OS doesn't seem like a great deal to be honest, considering you'll have to spend ~$100 on an OS.