When you say he won't disclose VIN does that mean he just didn't list it or you specifically asked him for it and he said no?
If he won't provide it to you, then that's a red flag because it means he won't allow you to do any research on the vehicle like a CarFax.
If he didn't list it, then that's not a problem. There are scammers out there harvesting VIN's. They take a known VIN and then put up a fake craiglist ad selling vehicle with that VIN. They take a potential buyer's deposit and run off.