Maybe you could rent the second installment of "Roots" or a similar film which shows historical depictions of slave auctions. Granted, there aren't any halflings in "Roots," but it should at least give you an example.
Slavers make their slaves wear makeup (especially over bruises and scars) to make them look healthy.
Have a variety of creatures. Halflings for agile slaves; and orcs, hobgoblins, etc for heavy load-lifting.
Slavers aren't good people. They may be charismatic like Bill Cutter, or they may be thugs, but either way, make sure that the PCs know that this isn't a good guy. Slavers never treat their charges with respect. They have to keep the slaves cowed, either through force or by belitling them.
The slaves should all have broken spirits. A defiant slave is a poor investment, but a cowed slave does what he is told because he has no will to refuse. Most people who have been slaves for some time may even refuse to leave if offered freedom, because they are afraid to leave. Slaves are pitiful.
Usually, in a slave auction, they would have the slaves line up at the back of the stage, and one by one, they would be brought forward. You might want to sort them by type(age, gender, size, race, etc) or you may want to have the slaves be chosen randomly. Occasionally, you could have a rich person who wants a number of slaves meet with the slaver to get the cream of the crop, but he'll be paying top-dollar for them.
