If you are buying and selling cars, you need to get car insurance that will cover this. As long as you are not dealing in high performance vehicles, this should not cost too much if you shop around. With this insurance, you will be able, for example, to tax vehicles and drive them legally. You also need to notify the tax authorities and should submit an annual tax return but you should make contact with them and use their advice line. www.hmrc.gov.uk
Yes, as a trader, you should fill in the trader section of the V5 when you buy a vehicle and the appropriate parts when you sell a vehicle. Remember that the details you put on the V5 go to a government department so if you do not want to get into trouble, you should follow the advice in the first paragraph as well.