Be careful.
I'd want to know why the seller doesn't have the title. I'd want to know if the seller was the last person to register the bike. Does he have the registration or other legal paperwork to prove he once had it registered in his name?
If the bike was once registered to him, he can get a duplicate pretty easily, and I think it's only $11.
There are companies like Bennett's Auto Title Services who will go through the hassle of obtaining hard to get titles. If you explain the situation to them, they will let you know if they can get the title or not.
99% of the time, unless you are using the bike for parts or racing, it's not worth the hassle. You only know what the current "owner" tells you, and I tend to be a bit leery of such transactions.