All cars over three years old need to have a valid Mot to be able to drive on the roads in normal circumstances (e.g. not driving to / from a test station). The MOT test itself usually costs between £35 and £55 pounds but if it fails, it could cost thousands to repair depending on what car it is and what it failed on.
Some people do try and sell cars with little or no MOT but this is usually for a reason. If they think the car may fail the MOT then they will often not want to put it in as the failure can be found on the internet by anyone with the reg number. If they are confident it will pass then they would probably put it in for the MOT as they are likely to make more than the cost of the test if they were selling the vehicle with 12 months MOT rather than none.
If when you go to look at a car you find it only has a month or so left on it then you could say that you will buy the car if they will put it through the MOT and it passes.