Hi , you will find a lot of cars sold by companies like those , are cars you would not be sold by a " propper" garage . A large number will be ex- fleet / or have hard to prove history and previous lives, be "grey" imports or even worse ! If you do buy from one of these outfits , it is essential that you get a full inspection done by either the AA or RAC ... it will cost a few hundred £'s but could save you a lot of money and or stress and problems in the future.The faults that the inspection WILL find may also enable you to negotiate on the price or even walk away from a "Nasty "
You may be told by them that the cars are AA or RAC approved , but these checks will have been carried out by the garage itself , so can in some cases be worthless.
I have been in the motor trade for 20 yrs , and i have come across many people who have had problems with "supermarket " car centers . I remember several years ago one person was sold a write off that had rebuilt and was a death trap .... that company has now gone out of business !
Also remember that if you do have a problem with the car , you will not get any back up or help from them , and the warranty they give will have very limited cover ..... but they will tell you otherwise !!!!
Good luck !