Witness a slave auction. Who could have stopped the auction?

Most of the people at the auction were not able to stop the selling, although they might have wanted it.
for example the clergy man:
He would have been thrown out of his church and he didn't want things to end like this.
Another good example is the seller, because we think he wouldn't have tried to sell Mariah and Samuel, if he didn't need money.
Of course Mariah and Samuel weren't able to escape the auction either, so the only option left for them was to run away.
If they had had money, they might have been able to buy themselves free.
They also could have run away, but they would have been caught and killed.
So they didn't have a chance, either.
Another person at the spectacle was the bidder.
He could have left his idea to buy the slaves and could have let them go back to their family, but he needed them because he was used to having slaves around his house. Very good!