True they well put them through sometimes with clean MOT's when they might not have achieved it at an independent testing station.There are so many small dealerships that mot their own cars on site that sell old cheap cars that all have clean mot's. I'd always be wary!
When I was helping my sister find a car nearly everyone she went to look at they said "we will put 12 months mot on for you" now that straight away just sounds dodgy to me as I would rather it have one present to check for advisories etc.
Someone I worked with bought a car for about 5k from a dealer so not exactly a £400 banger and it had a clean 12 months MOT despite the fact 2 of the Tyres were ******. one of them had a massive chunk out of the side wall that was downright dangerous