Its all about the balance of proof.... in your example, the first owner could pull out his original receipt and try to claim ownership of my car, but I could then pull out the sales receipt I have (bought from auction). The balance of proof is in my favour though - he has never reported "his" car stolen, he filled out the v5 passing responsibility for the car to someone else and I have a valid receipt, the car is registered to me, kept at my address and I have plenty of proof of servicing parts, fuel etc.
Even without a receipt, the balance of proof is still in my favour by a long way.
Not having a receipt doesn't mean you don't own it. I don't have the receipt for my telly any more, but its still MY telly.