I know some companies are actively approached by some brokers, but most are not, because a startup often dies as a zombie (= not in a big bang, in most cases).
BuyMyDeadStartup is a bulletin board, not a marketplace. A founder posts a text description of what the company left behind, verified against a working email address and read by a person before it goes up. Buyers write to the founder directly.
Whatever they agree is between them.
I really tried to keep this as simple as possible.