If you have bought a vehicle that had a previous owner that was disabled, then they will have a disable road registered on the log book. This is so they can benefit from paying less money for the tax, than we do. If you have bought a car and the owner is disable, if the disabled owner is going to use their disable tax disc on a another car in their name, then they will have the right to remove it from your car. Even if you keep it, you still can't use it and if you get caught, then you are highly likely to pay a fine and have points on your driver's license because you 're not disable and not eligible using it, so it's just to worth taking the risk. If you get caught using it by the Police, it will end up costing you more than getting a normal tax disc, so thinking about it wisely.
To get around this you have to change the disable tax class to normal, so you can get a brand new tax disc that is for private use. You can't go to the Post Office to change the tax class, i was in this situation and i have already tried it. From my experience, you will have to directly go to the nearest DVLA office and bring all your vehicles documents to change it. Then you can retax the car as a normal driver. Just to let you know that it will be more than a disable road tax, so don't get a shock, if the disable owner says he paid less, when you were speaking to him.