Sorry, never bought one. But I have bought a car and it was the responsibility of the seller to provide the paperwork, not the buyer.
When you go to register the vehicle, the guy behind the counter should give a list of any "Impuesto sobre vehiculos" is outstanding. If they are, the seller should pay them at the Ayuntamiento. He should also tell you if there are any outstanding fines on the vehicle in which case the seller should pay them at the correct office before you purchase. There may also be an embargo on the vehicle (not paid for, previous sellers documents wrong or whatever) in which case, say goodbye to the seller and look elsewhere.
I don't know where you'd get a copy of the "Permiso de circulacion" (log book) but would suggest this should be for the seller to sort out and not you.
I'm assuming the procedure is the same for a motorbike as it is for a car. Any outstanding taxes, fines etc become your responsibility. No correct log book in the name of the seller and he could take the vehicle back claiming he didn't sell it.
Can you not find a similar one from someone who has got the paperwork?