If you are looking to buy a used car or van then one of the musts to check is how much MOT is left on it. For vehicles over 3 years old, an MOT inspection must be carried out every 12 months. IF the car or van fails the Mot then the issues need to be rectified and a retest carried out before a valid test certificate is issued.
If buying from a garage they will almost certainly sell the car with at least 3 months MOT if not often even 12 months, but private buyers can be different.
Some people will sell their used van or car with only a few weeks/ days or even no MOT on. The price often reflects this but sometimes the price is only reduced by £50 or so. Often the advert will state “No MOT (or short MOT) but I do not think it will take much to pass”. This statement is widely used and often the seller knows that that is not the case. With an Mot costings as little as £20, it would be worth while anyone putting an MOT on to a car to sell it, therefore increasing the value, if they are not willing to MOT it then it would suggest they are aware of costly problems.