@OP - It's pretty simple. Some people have inflated ideas of what their used gear is worth. So they hold out for the price they think they should get and when they fail, they give up and sell to an online used goods retail business.
It's the fairytale of the Princess and the Pauper applied to Hi Fi.
As for TMR, they are effectively buying your gear at trade price so that they can apply a margin to it. They wouldn't have a business otherwise.