You are looking at slaves as if they are a freely and regularly traded commodity on the whole across the market and that there would exist a means of making a margin on said commodity, Mr Hyasyoda.
This is not the case. Amarrian slaves, on the whole, do not change hands overly often and tend to stay from generation to generation with the same Holder family, or at most a handful of Holder families through that particular slave’s bloodline. This is intended within Amarrian society as slaves are meant to be working off the sins of their forebearers while also being educated in the faith. A Holder would be disrespecting their duty if they treated their slaves purely as a commodity to margin trade or exchange at a frequent rate.
The listings for slaves on the Capsuleer market is a drop in the ocean compared to the general slave population and often serves as a means of offloading excess stock, so to speak, by Holders and/or organisations. The reasons for this are many and varied, from attempting to wash one’s hands of more troublesome slaves, trying to liquidate them due to urgent financial reasons, or simply looking to make an unscrupulous profit without any care of the end owner.
Trying to buy out slaves, or end slavery, in some kind of SCC market manipulation is a prime example of crying out into the wind.