You would need to see a reciept for thesale of the car and also some Identification that the seller is who he claims to be.

Remember a V5C is not proof of ownership, It is just proof of the registered Keeper.

If you are hoping to p-x a vehicle, buy from a trader who seems (or you know) to be reputable. If you plan to sell privately, look for a seller who seems as honest as yourself.

There are genuine sellers out there. A good bet is one who has owned the car for a few years and can show you a good set of paperwork.

I agree with other posters who are suggesting that someone who is as inexperienced as the OP seems to be should consider buying from a reputable dealer. Even this can be far from straightforward - there are rogues here as well as in the private sector, but at least the Sale of Goods Act provides some back-up.

When buying privately, there is little legal back-up, unless the car has been specifically mis-described - in an advert, or by word of mouth (difficult to prove).

You are in effect on your own. A friend with good mechanical knowledge, and some specific knowledge of the weaknesses of the particular car you're interested in, would help. Your own "sixth sense" and ability to judge people also. You save money over what you would spend at a dealer, but you should set aside the difference to cover the distinct possibility that you will need to rectify something fairly important.

Though I have bought cars privately in the past I wouldn't do it now - I think there's more dishonesty around. Sell privately - yes. A much better option than part-exchanging.