A relative of mine bought a garage-find motorcycle from the '70s in Michigan for $100. The title was long-gone, and the bike had been inherited and passed along with no documentation for years. He was able to title it after certifying with a guaranty agency for a small fee. Call your state's DMV and ask what it will take to purchase and register a really old vehicle with only a bill of sale.
If you're looking for old stuff, this will likely happen again, especially since as some previous posters said, some vehicles may have never had titles. Others survived an era when old vehicles were worse-than-worthless and people wouldn't have made a point to protect the paperwork. The good news is you will be the buyer who knows what is and isn't possible, and can also be the buyer that can offer a lower price, when others are afraid to offer at all.
On a side note, I would only do this with genuine vintage vehicles. Newer vehicles with missing titles might actually be scrap-titled somewhere, where information sharing between states will render the vehicle un-registerable. Good luck with your classic car search!