It can work both ways. If you start off an auction with what's very obviously going to be the best product, then people who missed out on buying it are going to bid as much as they can to avoid losing out on the next best products in the remaining sales. It gives off the impression to buy now since there isn't anything else coming up that's better. On the contrary, ending an auction with the best product makes people bid cautiously on all the products that came before it. However, people will be walking out the door talking about the best product like it's free advertising, increasing the notoriety of the auction house. I think it's pretty established that the slave auction here is widely known already, so there's no reason to end the auction off with the best slave.