Ancient auctions in Rome were mainly for the purpose of selling slaves. People that were captured from other countries during their many conquests would be put up for sale no different than if they were a piece of furniture or other wares to be sold. Those who would come to one of these auctions would inspect the slave up for sale probably no different than someone who goes to a car auction and examines the car. Just like cars slaves came with guarantees and if the slave had something wrong with them the seller would have to buy the slave back. They had six months in which to do this.
Those that bought slaves were usually Romans of high standing but on the other hand those that sold them were not thought very highly of much like what a pimp is considered to be like in today's standards. The slave traders also did not care how much they degraded the slaves in fact they would generally have the slaves stand totally naked in front of the crowds so that the buyer could see what they were getting. However, if a slave was exceptionally beautiful, they would not be put up for sale in a public market, special auctions would be held for beautiful slaves and the bidding would be done in private.
Slaves were kept in wooden or iron cages until they were needed for auction. They would be threatened with the whip and would be hit with a whip if they would not obey. If the slaves were from a different country they needed to be marked with white chalk on one foot so that the buyers would know that they were captured from a different country. The slaves would be fully inspected even down to their teeth before being put on the auction block. They would then place a plaque around their neck that would give the details of who they were, where they were from, their good points, their bad points. This was required by Roman law.
During the auction the slaves were displayed so that all could see them and sometimes they were on a revolving stand. A lot of them would be stripped of their clothes. Even if they came out in clothes a possible buyer could demand their clothes be torn off so they could see what they were buying. If the slave trader had not figured out anything about the slave and couldn't give any guarantees then the slave had to wear a special cap on their head that alerted buyers to this situation.
Buyers would discuss the body of the slave in front of everyone as if they were inspecting the engine of a car and they could examine each and every part of the body they felt necessary in order for them to make a decision on whether they wanted to buy them or not.
The amount of money paid for any slave differed. It all depended upon their age, the skills they had and other qualities. For instance if a slave was well educated they would cost about twelve times more than a slave that had no education. Women slaves, unless extremely beautiful would cost far less than male slaves. Romans also loved novelty so if a slave was a dwarf or was mentally handicapped for instance would bring in a lot more money than other slaves.
Note: Prisoners of war usually had a different fate and that was that they would be forced into being gladiators and the only way to continue to survive was to continue to win their battles.