Probably certain markets for beautiful, healthy slaves, athletic males etc.. would only have the wealthy attending the auction because they are the only ones that could afford them. Common laborers, mine workers etc. probably would not see those kind of buyers there. An estate steward would be there.And this brings up another point with a slave sale. How hard is the slave trying to sell themselves?
Slavery is an extremely harsh and dangerous existence and even the newly enslaved will know this. Now brand new slaves were probably more defiant and looking for a way out. A slave with some mileage on will want to land in the best situation possible and if a woman, even if not being sold for sexual purposes, will understand her sexuality will be her best weapon. Just like when you are trying to send signals to a potential liaison in a bar you would try to send signals to certain buyers (assuming an auction).
You WANT to be bought by the guy with the nice cloths and jewelry (even if you never see him again a Roman estate was an industrial complex which means the resources to take proper care of you) not the guy in the butchers apron (which probably means had work, you are last in line for food and cloths and you will spend a lot of time under him keeping his bed warm).
So sellers be aware that how she comes on to you has NOTHING to do with you. It has to do with how successful and rich you look. LOL