They don't send reps round, that would make it too easy for you to reject their offer when you are at home.
They give a good price online, then you have to take the car to one of their depots (usually back end of nowhere with no public transport links) with a friend who can take you home as you will have expected to sell your car. Then they tell you about all its faults and significantly reduce the amount offered. As you've emotionally sold the car, have inconvenienced your friend and you're pissed off with the process, you accept their offer, just to be shot of the thing.
I should point out that's no reflection on any particular organisation but about the way used car dealers make their money.