Was a bit distracted by those lovely ladies being fist fucked. And, no, no clue what will haappen next. Will they be whipped?

Back to the slave auction, you have a point indeed. All the free sex around.. Charity sounds like a great idea. Maybe for a girl orphanage? Am quite sure, by the way, that some buyers would love to acquire a few slaves for use at home. On the action, as you ask, was thinking of:
- girls standing on the podia to be shackled on ankles and wrists, plus collared, some ball gagged
- a special lbox with clamps, needles, dildos, whips, floggers, electric shockers is given as a bonus for every girl sold. The girls are made aware of the content of this box, adding to their excitement
- as a starter one girl is being tied standalone, hands above her head, shackled to a chain from above, being whipped. This is to arouse the audience and increase the sales price
- midway thenauction another girl is taken to this place in the middle of the auction, chained likewise, raised in the air. A pole with a huge dildo is positioned below her and she is slowly lowered onto this dildo as the bidding around her continues

Hope this provides further inspiration to add to your artistic talents!