Warning: Story contains discussion of abuse and rape
When they say a car loses a certain percentage of its original value the moment you drive it off the lot, what they mean is that the value of a car decreases in the years you own it. The same is true for slaves and, just like it is with cars, the rate at which a slave's value decreases is determined by a number of factors.
One of these factors is the slave's age. Slave children are usually allowed to remain in their parent's care until the age of 14, if the slave's owner doesn't decide otherwise. Slave traders are obligated to sell children with their parents, although, to be fair, no one really investigates whether or not they really do. Then again, no one ever seems to investigate anything concerning the treatment of slaves. Slaves are considered most valuable in their teens and early twenties. After that, their value decreases rather rapidly, but since slaves hardly ever live to their 40's, this drop in value hardly concerns anyone.
Other factors such as skills, strength, obedience, training and experience are factors that aren't as easily quantifiable and are usually assessed by the slave traders for each slave individually.
The most important factor, however, is the purpose a slave is used for. Slaves that are used for hard labour may expire rapidly if they are not well taken care of, but, if treated well, a slave's value may even increase as they gain strength and skill.
This does not apply to pleasure slaves. Pleasure slaves are the most frequently re-sold type of slave, since their owners usually get bored of them rather quickly, which makes the pleasure slave market one of the biggest slave markets there is.
Beauty is a curse for slaves. Only the prettiest of slave children are taken to be sold as pleasure slaves, usually at an age as early as 12. From the moment their virginity is taken, their worth only decreases. The first time a pleasure slave is re-sold their price will be somewhere around 50% of the price they were originally sold for. After the second owner they'll be worth 25% of their original price.
To inform their customers about the number of owners the slaves have had, slave traders will often arrange their slaves in lines under banners that say "NEW", "50% off", "75% off", "87,5% off" or "93,75% off". Each slave has to undergo a medical examination before they can be re-sold. If a pleasure slave contracts a sexually transmitted infection or disease, their worth immediately drops to zero. The costs for medical treatment usually exceed the slave's value so slave traders usually don't pay for treatment. Those slaves are either 'put down' immediately or put under a sign that says "100% off" as an indicator to their predicament, they can be bought for miniscule sums, though they usually aren't since no one ever choses a diseased slave over a healthy one. A healthy slave with 3 previous owners is easily affordable as well, which is why most traders don't even bother announcing to the world that they even have a diseased slave in their lot and just get rid of them right away.