Is he selling a note with seller financing? Doesn't make sense, unless he's selling a partial but you wouldn't call it seller financing.
If it's a home and the numbers work, go for it. If he's an intelligent investor, he may be reluctant to give you personal info, other that perhap DL and introducing himself. I'd certainly not divulge my assets to ANYONE.
I do a fair amount of seller financing.... as a buyer and as a seller. I've never once been asked for ANY identification ever. And I've never had a bad deal or a pixx off customer. Yes, I"ve passed on a few, they all don't work.
Talk to the guy, get to know him, look at the property, look at the numbers. If you're uncomfortable with him, pass, regardless if it's good or not. There ARE people out there that you just don't want to deal with.
However, he could be a great guy that you might end up doing several deals with him, so check it out.