I know some states don't issue titles on cars that are older then a certain age. If the buyer lives in a state that does require titles then I suppose it is up to the buyer to straighten that issue out. That sucks.
Yes, the requirements are typically outlined and the seller should be willing to provide help with the documents.
All states are different, but a buddy went through this (Georgia car to Ohio). He had to have a bill of sale, copy of the registration (the car was being used therefore licensed) proof that is was insured and of course an inspection in the new state (just looked for the manufacturers' VIN number in the correct place).
The seller was cooperative and it all went easy for him. Other states or title bureau offices may not be so easy.