I must emphatically assure my own audience that I am unconvinced that this approach is viable. Although some passages in Leviticus could perhaps convict its author (e.g. Lv 25:44-46), I have never encountered a cohesive argument against Biblical inspiration which depends on modern moral condemnation of slavery. Since Scripture never explicitly denies the evils of slavery, we have some limited room for subjective interpretation. Acknowledging that fact constitutes the best defense I know from skeptics' criticisms, and indeed it may be adequate to the task. Christians themselves often contend that the Bible in no way implies God condones the institution of slavery.
That said, Christian apologists do sometimes take a different approach with which I have serious issues, claiming that the slavery of antiquity was not really so horrible, and that we should therefore not expect God to vehemently object to it. For example, Michael Marlowe, a Reformed theologian, Pittsburgh Theological Seminary alumnus, and editor of the website bible-researcher.com, describes ancient slavery thusly:
Those who have been in the military have experienced something like it---being legally bound to an employer and to a job that one cannot simply "quit" at will, not free to leave without permission, subject to discipline if one disobeys or is grossly negligent---all of this is familiar enough to those of us who have served in the military. And yet we know that the daily life of a good soldier is not especially hard. This is what it was like to be a slave.
Of course, this is patently false. Modern military service is not remotely similar to slavery in ancient cultures.
In order to justify my disagreement, I shall present evidence in the form of selected quotations of ancient literature on the subject of slavery. Please note that I do not indent to exhaustively catalog antiquated opinion, but rather provide a small sample for the reader of the much larger evidentiary case for our historical understanding of the institution as it was practiced so many years ago.
We may begin with Aristotle, who believed that some men were naturally slaves, and that the universality of slavery was therefore an inevitable feature of the natural order of things. He wrote at great length on this topic in his work Politics, of which I reproduce one particularly disturbing passage here:
Those men therefore who are as much inferior to others as the body is to the soul, are to be thus disposed of, as the proper use of them is their bodies, in which their excellence consists; and if what I have said be true, they are slaves by nature, and it is advantageous to them to be always under government. He then is by nature formed a slave who is qualified to become the chattel of another person, and on that account is so, and who has just reason enough to know that there is such a faculty, without being indued with the use of it; for other animals have no perception of reason, but are entirely guided by appetite, and indeed they vary very little in their use from each other...
If these sentiments were common, as is reasonable to suppose, then we gain insight into how we might expect free citizens to treat their nation's bonded population. In one account, for instance, recorded by Diogenes Laërtius, we hear that the philosopher Diogenes the Cynic (of no relation to his biographer) was enslaved by pirates and sold on the Cretan market, whereupon he insisted that he was skilled in "governing men." Accordingly, he was put to work tutoring the sons of one Xeniades, resident of Corinth. On this occasion, then, we observe that the ancients acted in concordance with the belief that some men were naturally predisposed to a particular service, in this case tutorage. Diogenes Laërtius also records that another fourth-century Greek philosopher, Phaedo of Elis (after whom Plato's famous dialog was named), was similarly captured and "compelled to submit to the vilest treatment" before he was freed by Socrates.
Aristotle's own tutor, Plato, likewise penned extensive thoughts on slavery, and described a great variance of treatment to which slaves in his day were subjected. Quoting an earlier poet, he writes:
Different persons have got these two different notions of slaves in their minds---some of them utterly distrust their servants, and, as if they were wild beasts, chastise them with goads and whips, and make their souls three times, or rather many times, as slavish as they were before;---and others do just the opposite.
So we see that the comparatively comfortable employment of Diogenes with respect to Phaedo's "vilest treatment" reflect typical snares of fortune in the ancient Greek slave industry.
Aristotle characterizes the "rule...of a master over slaves" as "despotic," further confirming our modern understanding of slavery as great evil, and suggests tantalizing slaves with the possibility of freedom, presumably in order to manage their behavior and compel them into docility. In the same book he quotes an earlier proverb, affirming that "there is no leisure for slaves." Indeed, prior to the sixth century B.C., loans in Athens had been "secured upon the debtor's person" by "custom," and if the debtors "failed to pay their rent they were liable to be haled into slavery, and their children with them." This evidently widespread problem persisted in Athens prior to the election of Solon as Archon c.594 B.C., and highlights the capriciousness of the Athenian slave industry.
Scripture itself affirms the evils of slavery in its depiction of Moses, Aaron and the plight of the Hebrews in Egypt. Consider the haunting passage from Exodus 1 (ESV), in which the Egyptian's treatment of the Hebrews is tersely stated:
13 So they ruthlessly made the people of Israel work as slaves 14 and made their lives bitter with hard service, in mortar and brick, and in all kinds of work in the field. In all their work they ruthlessly made them work as slaves.
In addition to all this, we may further recall that bondage passed from parent to child, such that children of slaves were themselves born into slavery. This practice is referenced in Lv 25:46, in which slave lineages are promised to Jewish generations "to inherit as a possession forever" (ESV); yet it was by no means confined to the near east. We may consider the Helots, an entire slave population in ancient Sparta, kept subdued up to at least the fifth century B.C., and whose mistreatment meets any modern horror. For example, Plutarch records an ordinance given by the eighth-century Spartan legislator Lycurgus, establishing the custom of indiscriminate murder of Helots by young Spartan men. So objectionable was the Spartan persecution of their Helot slaves---and other lower classes, for that matter---that we have preserved for us by Xenophon the observation that "whenever among these classes [Helots, freedmen, lesser Spartiatae, and Perioeci] any mention was made of Spartiatae, no one was able to conceal the fact that he would be glad to eat them raw."
As previously indicated, I am disinterested at this time in taking sides regarding the moral argument against Biblical discussions of slavery; rather, I aim only to address one particular apologetic claim. To this end, I have provided here a few scattered quotations in the ancient literature regarding the slave industry and the treatment of those bonded, not to paint a complete picture of the institution in antiquity, but rather to dispel the modern misconception among Christians that slavery was somehow humane in those days. I can find no basis whatsoever in the historical record to justify such a bizarre suggestion, which appears motivated by theology rather than a primary concern for historical accuracy.
 Marlowe, Michael. "Some Observations on Biblical Interpretation and Slavery" (an internet essay). http://www.bible-researcher.com/slavery.html
 Aristotle, Politics: A Treatise On Government I.V, tr. William Ellis, A.M. http://www.gutenberg.org/files/6762/6762-h/6762-h.htm
 Diogenes Laërtius, Lives and Opinions of Eminent Philosophers VI.II.IX (Diogenes of Sinope), tr. Charles Duke Yonge. http://classicpersuasion.org/pw/diogenes/dldiogenes.htm
 Diogenes Laërtius II.IX.II (Phaedo). http://classicpersuasion.org/pw/diogenes/dlphaedo.htm
 Plato, Laws VI, tr. Benjamin Jowett. http://classics.mit.edu/Plato/laws.6.vi.html
 Aristotle, Politics VII.III,X, tr. Benjamin Jowett. http://classics.mit.edu/Aristotle/politics.7.seven.html
 Aristotle, Politics VII.III.XV.
 Aristotle, The Athenian Constitution I.II, tr. Sir Frederic G. Kenyon. http://classics.mit.edu/Aristotle/athenian_const.1.1.html
 "By this ordinance [the Cryptia], the magistrates despatched privately some of the ablest of the young men into the country, from time to time, armed only with their daggers, and taking a little necessary provision with them; in the daytime, they hid themselves in out-of-the-way places, and there lay close, but in the night issued out into the highways, and killed all the Helots they could light upon; sometimes they set upon them by day, as they were at work in the fields, and murdered them." Plutarch, Lycurgus, tr. John Dryden. http://classics.mit.edu/Plutarch/lycurgus.html
 Xenophon, Hellenica III.III.VI, tr. Carleton L. Brownson. http://old.perseus.tufts.edu/cgi-bin/ptext?lookup=Xen.+Hell.+3.3.1